JP7675549B2 - Light monitoring and control system, master station, slave station, and communication method - Google Patents

Description

An embodiment of the present invention relates to a light monitoring and control system, a master station, a slave station, and a communication method.

Light monitoring and control systems are essential for the safe operation of aircraft taking off and landing at airports. At airports, a wide variety of lights, such as guide lights and control lights, are installed on runways and taxiways. In many systems, the power lines that supply power to the lights also serve as the communication medium for monitoring and controlling the lights. This type of method is called power line carrier. Power line carriers are widely used in stop line light systems (STBLs), which turn lights on and off to guide aircraft on the ground, runway status light systems (RWSLs), and for detecting broken light cores.

In existing airport facility monitoring and control systems that use power line carriers, commands related to lighting monitoring and control are generated by changing the signal injection pattern into the power line. Power lines are often laid over distances of several to tens of kilometers and are unshielded, making them an extremely harsh environment for communication. For this reason, commands were previously created by lining up one bit indicating whether a signal is present or absent, which was not necessarily responsive or versatile.

For example, there is known technology that can realize communication using high frequencies of 10 kHz to 150 kHz using orthogonal frequency division multiplexing (OFDM) by constructing a dedicated power supply circuit. This technology can increase communication rates, but it comes at a high cost. Also, in Japan, restrictions under the Radio Law make it difficult to use such high frequencies, especially near aircraft.

Patent No. 4008693 US Patent Application Publication No. 2005/0253929

In a lighting monitoring and control system using power line carriers, it has been difficult to speed up the control response due to the harsh communication environment. There is a demand for technology that can withstand adverse conditions, improve responsiveness, and realize lighting control without increasing costs.
Therefore, an object is to provide a light monitoring and control system, a master station, a slave station, and a communication method with improved responsiveness at low cost.

According to an embodiment, the light monitoring and control system includes a parent station and a child station. The parent station communicates with the child station via a power line that supplies power to the airport lights. The child station is connected to the power line via a transformer. The primary side of the transformer is connected to the power line, and the transformer supplies power extracted from the secondary side to the lights connected to the child station. The parent station includes a modulation unit and an injection unit. The modulation unit generates a modulated signal by modulating a carrier signal with a multi-bit code using a modulation method that represents a symbol by a combination of frequency shift and phase shift. The injection unit injects the modulated signal into the power line in synchronization with the power supply cycle of the power. The child station includes a receiving unit and a demodulation unit. The receiving unit receives the modulated signal from the power waveform on the secondary side of the transformer. The demodulation unit demodulates the received modulated signal to reproduce the code.

FIG. 1 is a diagram showing an example of a light monitoring and control system according to an embodiment. FIG. 2 is a functional block diagram showing an example of the master station 3. As shown in FIG. FIG. 3 is a functional block diagram showing an example of the slave station 6. As shown in FIG. FIG. 4 is a diagram for explaining a modulation method in the embodiment. FIG. 5 is a diagram showing an example of a signal bandwidth. FIG. 6 is a functional block diagram showing the demodulation process in the slave station 6. As shown in FIG. FIG. 7 is a diagram showing a change in phase. FIG. 8 is a diagram for explaining the phase determination. FIG. 9 is a diagram for explaining synchronization with a reference signal. FIG. 10 is a diagram illustrating an example of a communication sequence. FIG. 11 is a diagram for explaining an increase in the child station signal.

Hereinafter, an embodiment will be described with reference to the drawings.
Fig. 1 is a diagram showing an example of a light monitoring and control system according to an embodiment. In Fig. 1, a power source (CCR) 1 installed in a power station building of an airport is connected to a power line (light circuit cable) 8 via a bypass filter 4. The power line 8 connects the primary sides of a plurality of rubber transformers 5 in series. A slave station 6 is connected to each secondary side of the rubber transformer 5. Furthermore, a light 7 is connected to each slave station 6. The rubber transformer 5 ensures isolation between the power line 8 and the slave station 6, but power from the power line 8 is supplied to the slave station 6 and the light 7, so that a constant current flows to each light.

The bypass filter 4 is installed, for example, near the power source 1, and separates noise in the communication route of the power line 8 from power source noise. For example, the current flowing through the LRC resonant circuit is obtained and refined by a filter to obtain a commercial power waveform. As shown in FIG. 1, by inserting the bypass filter 4 between the power source (CCR) 1 side and the power line 8, the signal frequency bands used by the power source 1 and the lamp 7 are separated. That is, an LRC resonant circuit is constructed with L (coil), R (resistance), and C (capacitor), and the signal used on the lamp circuit side circulates on the lamp circuit side, and the CCR noise circulates on the power source side.

Furthermore, the master station 3 is connected to the bypass filter 4. The master station 3 functions by receiving power from the power source 1, and communicates with the subordinate slave station 6 via the power line 8 by power line carrier. The power source 1 may be multiplexed into a normal power source 1-1 and a standby power source 1-2. In addition, a stabilization device (OP/C) 2 may be provided between the power source 1 and the bypass filter 4.

The master station 3 is accommodated in the master unit 10 via a control LAN (Local Area Network) 9. The master unit 10 is a robust computer, such as a PLC (Programmable Logic Controller). The master unit 10 is further connected to a monitoring console 11 via the control LAN 9. The monitoring console is, for example, a general-purpose server computer, and is equipped with a monitor that displays the runway conditions in real time.

In FIG. 1, the parent station 3 transmits, for example, a query command periodically to the subordinate child stations 6 connected to a common power line 8. The child stations 6 transmit a response signal to the parent station 3 at a timing according to the address assigned to them. The child stations 6 monitor the status of the lights 7, and notify the parent station 3 of any malfunctions such as a burned-out bulb. This allows the location of the broken core to be immediately detected and displayed on the monitor of the monitoring console 11. The operator can refer to this display and take appropriate measures.

Similarly, in a stop line light system or RWSL system that controls lights, control of lights 7 is performed by power line carrier communication using power lines 8. Based on information acquired by an aircraft detection sensor or ground radar, control commands are given from parent station 3 to child station 6, which controls the ON/OFF of subordinate lights. This makes it possible to realize ground guidance of aircraft by controlling the lights.

Figure 2 is a functional block diagram showing an example of a parent station 3. In Figure 2, the power line on the IN side in relation to the power source 1 is called the F side, and the power line on the OUT side is called the R side. Signals from the child station 6 are received at two points, the F side and the R side. In other words, even if the reception level of one signal is low, a diversity configuration is adopted that can compensate for it with the other. In the embodiment, the signal is injected into the power line 8 on the F side.

The parent station 3 includes a signal processing unit 31, a processor 32, interface units 33 and 38, a waveform shaping unit 34, an amplifier unit 35, a signal injection unit 36, and a power supply waveform acquisition unit 37. Among these, the power supply waveform acquisition unit 37 digitally converts the power supply waveform acquired from the bypass filter 4 and passes it to the processor 32.

The signal processing unit 31 includes a modem unit 31F and a modem unit 31R. The modem unit 31F generates a modulated signal by modulating the carrier signal on the F side with a multi-bit code using a modulation method that represents a symbol by a combination of a frequency shift and a phase shift. The generated modulated signal is input to a signal injection unit 36 via a waveform shaping unit 34 and an amplifier unit 35. The signal injection unit 36 injects the modulated signal into the power line 8 in synchronization with the power supply cycle of the power source 1.
The modem unit 31R generates a modulated signal by modulating the R-side carrier signal with a multi-bit code using a modulation method that represents a symbol by a combination of a frequency shift and a phase shift.
In the embodiment, a modulation method is adopted in which a two-bit code is associated with four symbols represented by two different frequencies and phases, which may be called a dual modulation method.

The processor 32 provides a multi-bit code including a command to be sent to the slave station 6, the address of the master station 3, etc., to the signal processing unit 31 to generate a modulated signal. The processor 32 repeatedly executes a transmission/reception sequence including the transmission of a code from the master station 3 to the slave station 6 and the transmission of a response code from the slave station 6 to the master station 3.

The modem unit 31F demodulates the signal waveform extracted on the F side and reproduces the response code from the slave station 6. The modem unit 31R demodulates the signal waveform extracted on the R side and reproduces the response code from the slave station 6. The reproduced code is passed to the processor 32 for processing. Various data included in the response code are then transmitted from the interface 38 via the control LAN 9 to the master unit 10.

Fig. 3 is a functional block diagram showing an example of a slave station 6. The slave station 6 includes a processing unit 6A and a power line unit 6B. As shown in Fig. 3, the slave station 6 is installed between the rubber transformer 5 and the light 7. The power line unit 6B is connected to the secondary side of the rubber transformer 5, supplies power to the light 7, and inputs a power waveform to a receiving unit 60 of the processing unit 6A. The state of the light 7 is acquired by a signal acquiring unit 67 and passed to a core breakage control unit 64 of the processing unit 6A. The driving power of the slave station 6 is acquired from the power line 8 by a power source acquiring unit 68.
An aircraft detection sensor 12 for detecting an aircraft is connected to the secondary side of the rubber transformer 5. The lighting side is short-circuited.

The receiver 60 of the processing unit 6A receives the modulated signal transmitted from the parent station 3 from the power waveform on the secondary side of the rubber transformer 5. The received modulated signal is input to a programmable logic device (PLD) 61. The PLD 11 demodulates the modulated signal and reproduces the code transmitted from the parent station 3. The reproduced code is passed to the processor 62.

The processor 62 processes the code from the PLD 11, the data from the broken core control unit 64, and the data from the aircraft detection sensor 12 via the sensor interface 63, and generates a response code to the master station 3. The response code is input to the PLD 61. The PLD 61 generates a response signal by modulating the carrier signal with the response code using the modulation method (dual modulation method) of the embodiment. The response signal is passed to the transmission control unit 65.

The transmission control unit 65 controls the transmission circuit 66 of the power line unit 6B to inject a response signal into the power line 8. This causes the response signal to be transmitted to the parent station 3 via the power line 8. In a particular embodiment, the transmission circuit 66 shorts/opens the secondary side of the transformer in synchronization with the power supply cycle to generate an overvoltage, and transmits the response signal using this overvoltage.

The transmission circuit 66 generates an overvoltage on the secondary side of the rubber transformer 5 by opening and closing the power supply circuit like a switch. By using this overvoltage as a signal, it becomes possible to generate a signal with low power consumption of less than 10 W.

Figure 4 is a diagram for explaining the modulation method in the embodiment. As shown in Figure 4 (a), the well-known methods are quadrature phase shift keying (QPSK), which assigns a signal to a phase every 90 degrees and codes it, and quadrature frequency shift keying (QFSK), which assigns four frequencies to each code. However, QPSK modulation requires a relatively wide bandwidth to determine the 90-degree phase. QFSK modulation requires the use of four frequencies, and if the frequencies are too far apart, some codes may or may not reach one signal due to the effects of standing waves. Either method is difficult to apply to power line carriers in airports, where the communication environment is harsh.

In this embodiment, as shown in FIG. 4(b), four codes are expressed by one signal by combining two frequencies (frequency shifts) and two phases (phase shifts). In other words, in this embodiment, different frequencies ch1 and ch2 are used (DFSK modulation) and two phases of 0° and 180° are used (DPSK modulation) to express four codes ("00"/"01"/"10"/"11") with one signal (symbol).

Figure 5 shows an example of a signal bandwidth. As shown in Figure 5, for example, if a 5 kHz signal is used and the signal width T is 1.4 ms, the band including the main lobe and one side lobe is 3.6 kHz (-2/T) to 6.4 kHz (2/T).

For example, if the power supply frequency is 50 Hz, a typical noise distribution on the power line 8 tends to be large at 20 times 50 Hz, 5000, 6000, and 7000, and is particularly noticeable at 4000 kHz. Phase modulation with a wide bandwidth is easily affected by noise, and even if the S/N ratio meets the specifications, there are cases where the phase cannot be read.

The total length of the power lines under one parent station 3 can be several kilometers to tens of kilometers. For this reason, the power from the power source is not received at the same level at all lights, and peaks and valleys in the level occur. This is the effect of standing waves. For example, even in a place (light) where the level is high at 5 kHz, the level may become low at 6 kHz. This phenomenon can occur even with frequencies 1 kHz apart, so in the case of QFSK modulation, four frequencies must be used. For this reason, if the frequencies are separated too far, codes will occur that may or may not reach one signal.

For example, even if a signal is sent at 100 Hz intervals, the difference obtained may only be the margin of error. If the signal width T is 1.4 ms, this is an interval of 1/T = 700 Hz or less, which is a narrower band than the main lobe of 4.3 kHz (-1/T) to 5.7 kHz (1/T), so there is no apparent difference. When sending 5.0/5.1./5.2./5.3 kHz, the transmission frequency is at its maximum, but it may appear as another frequency and only a slight difference may appear. For this reason, if spike noise of a specific frequency is introduced, it may not be possible to capture the frequency correctly. Ensuring a certain degree of power difference between frequencies can reduce the effects of external noise and ensure stable communication.

For example, if two frequencies are used with a frequency interval of 300 Hz, a full-scale difference of 5.0 kHz power: 5.3 kHz power = approximately 10:7 occurs. By generating a clear difference, more stable communication becomes possible. Therefore, in the embodiment, communication is performed between the parent station 3 and the child station 6 via power line carrier using two frequencies (DPSK modulation) on two channels and a modulation method with two phases of 0°/180° (DFSK modulation).

Furthermore, in the embodiment, multiple carrier signals with different bands are modulated by a dual modulation method to generate multiple modulated signals with different bands. That is, the modem unit 31F (Figure 2) modulates carrier signals with different frequencies F1 and F2 to generate a modulated signal for each band. The generated modulated signals are injected into the power line 8 by the signal injection unit 36.

Figure 6 is a functional block diagram showing the demodulation process in the slave station 6. The demodulation process is mainly executed by the PLD 61 according to a program stored in advance in a memory or the like. The power waveform received by the receiver 60 is input to the demodulators 61F1 and 61F2. The demodulator 61F1 performs demodulation process in the F1 band, and the demodulator 61F2 performs demodulation process in the F2 band.

As shown in Fig. 6, the two frequencies ch1/ch2 used in the F1 band are ch1: 5.0 kHz and ch2: 5.3 kHz. The two frequencies ch1/ch2 used in the F2 band are ch1: 6.0 kHz and ch2: 6.3 kHz. In the demodulation units 61F1 and 61F2, the signals of the two frequencies ch1/ch2 in the F1 band and the F2 band can be obtained in orthogonal phases of Q phase and I phase. When the signal power is represented by P, it can be obtained by P = {(Q phase) ² + (I phase) ² } ^1/2 .

In the embodiment, the phase shift between signals is determined based on whether or not there is a phase shift with respect to a reference signal. Communication between a parent station and a child station is performed by repeating one communication sequence (protocol) as a unit. In the embodiment, the first signal in the communication sequence is used as a reference signal, and the phase shift is detected with respect to the phase of the reference signal.

In the modulation method of the embodiment, the demodulation process involves comparing the magnitude of the channel power spectrum to determine which channel is being used, and encoding the data by determining the phase shift from the reference signal. Furthermore, encoding is performed by analyzing the waveform patterns appearing in the Q and I phases.

Figure 7 is a diagram showing phase changes. As shown in Figure 7, in a situation where the phase is inverted each time a communication sequence occurs, the reference phase also changes each time. Therefore, rather than using an absolute 0° as the reference, the phase is changed relatively and the phase relative to the phase of the reference signal is detected. This has the advantage of being able to operate without having to adjust for the variations that exist in individual communication stations.

The modulation method of the embodiment transmits reference signals in the F1/F2 reference frequency band and ch1/ch2 each time a command is executed. This type of transmission format allows communication to be established with short-term timing adjustment. In addition, by determining the phase shift with respect to the reference signal, the phase is determined relative to the reference signal, and each station (parent station, child station) has an absolute 0° reference, so there is no need to worry about variations between stations. This makes it possible for each station to perform phase determination.

8 is a diagram for explaining the phase determination. As shown in FIG. 8, the phase waveform can be determined by the sign that appears in the phase determination of each channel. That is, the following relationship exists.
Same channel, same phase: The signs of the reference and Q phase and I phase match.
- Same channel, different phase: The signs of the reference, Q phase, and I phase are reversed.
Due to these characteristics, by analyzing the code of each phase, it is possible to identify 0°/180° for each of ch1/ch2, and based on this identification result, it is possible to classify into four codes.

In other words, by transmitting signals from the parent station 3 to the child station 6 in two different frequency bands, for example F1 (ch1: 5.0 kHz/ch2: 5.3 kHz) and F2 (ch1: 6.0 kHz/ch2: 6.3 kHz), even if one frequency band cannot reach the child station due to standing waves, the other frequency band can reach it. This makes it possible to realize operation with increased robustness as a system.

Here, the master station 3 transmits two frequencies, F1 and F2, to the slave station 6, while the slave station 6 communicates with the master station 3 using, for example, only the F1 frequency. This simplifies the configuration of the slave station 6. In this case, by receiving at two points on the F side and the R side in the master station 3, even if the signal does not reach one side due to the effects of standing waves, etc., it is possible to capture the signal at the other side.

Figure 9 is a diagram for explaining synchronization to a reference signal. As shown in Figure 9, in each frequency band, parent station 3 transmits a reference signal for each communication sequence. Since child station 6 transmits only at F1, it replies at the parent station's F1 timing. When child station 6 receives a signal from parent station 3, it synchronizes its own signal with its own clock and replies. Similarly, parent station 3 determines the phase of the child station signal by timing it with its own (parent station 3) clock from the timing of its own transmission.

By providing a clock with common specifications in both the parent station 3 and the child station 6, pseudo-synchronous communication can be realized without the need to send a synchronization signal over the communication line. In other words, by using the head signal as a trigger and timing it with their own internal clocks, it is possible to create a state similar to that in which synchronization is established between the parent station 3 and the child station 6.

However, synchronization is performed on a per-command basis, and the timing is re-established each time a communication sequence is completed. The time required for this process is approximately a few power cycles, and it can be assumed that no excessive differences will occur.

In general, in a power line carrier system, deviations in timing between the transmitting and receiving sides can occur due to differences in how the power waveform appears at both the parent station 3 and the child station 6. This can cause the phase reference to shift, making phase modulation difficult. In this embodiment, the receiving station uses its own clock as a reference signal for counting the timing. This makes it possible to obtain processing timing without being affected by deviations that arise from differences in how the waveform appears.

Figure 10 shows an example of a communication sequence. The transmission format transmits ch1 and ch2 timing, the parent station address, and commands at frequencies F1 and F2, and assigns half a power cycle to a child station. By assigning a child station to a power cycle, the response time can be shortened. The circled numbers in the figure indicate the chronological order.

In this embodiment, the signal from the slave station 6 is composed of one symbol. Therefore, even if the output period is made longer than the signal width actually used, there is no risk of interference with other signals. The signal output from the slave station 6 utilizes the overvoltage generated by opening the secondary side of the rubber transformer 5, so the signal has the characteristic that the signal becomes larger as the signal length becomes longer.

Figure 11 is a diagram for explaining the increase in the child station signal. As shown in Figure 11, in the embodiment, a guard interval with a length of about 20% of the signal length is added before and after the signal section that is actually used. Since the signal often becomes distorted at the rising and falling edges, providing a guard interval not only increases the signal output, but also makes it possible to improve the accuracy of signal extraction at the parent station 3.

As shown in Figure 11, by emitting a signal before and after the normal section in which the slave station 6 actually uses the transmission signal, the signal of the slave station with signal output constraints can be made larger and more stable. This is because the overvoltage generated by opening the secondary side of the rubber transformer 5 is utilized. This allows for a larger signal volume and a more stable level, realizing higher quality communication, compared to generating the signal only in the required section.

As described above, in the embodiment, in an airport lighting monitoring and control system using power line carrier, communication between a master station and a slave station is realized by a modulation method that expresses four codes (00/01/10/11) using two frequencies and two phases. The ability to express four codes with one signal promotes the aggregation of information, thereby improving responsiveness. In other words, code information representing text can be injected into the lighting circuit, promoting the high level of informationization of signals and realizing faster control responsiveness. In addition, by using the power supply cycle as a synchronization signal, it is possible to simplify the signal configuration of the slave station signal, thereby improving noise resistance.
Furthermore, simply changing the communication method does not require special measures such as rewiring power lines or replacing power supplies, and existing power line communication facilities can be reused.

As a result, according to the embodiment, it is possible to provide a light monitoring and control system, parent station, child station, and communication method with improved responsiveness at low cost.

Although an embodiment of the present invention has been described, this embodiment is presented as an example and is not intended to limit the scope of the invention. This new embodiment can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the gist of the invention. This embodiment and its modifications are included within the scope and gist of the invention, and are included in the scope of the invention and its equivalents described in the claims.

1...power supply, 1-1...normal power supply, 1-2...standby power supply, 2...OP/C, 3...parent station, 4...bypass filter, 5...rubber transformer, 6...child station, 6A...processing unit, 6B...power line unit, 7...light, 8...power line, 9...control LAN, 10...parent unit, 11...monitoring console, 12...aircraft detection sensor, 31...signal processing unit, 31F...modem unit, 31R...modem unit, 32...processor, 34...waveform shaping unit, 35...amplification unit, 36...signal injection unit, 37...power waveform acquisition unit, 38...interface, 60...receiving unit, 61F1...demodulation unit, 61F2...demodulation unit, 62...processor, 63...sensor interface, 64...core breakage control unit, 65...transmission control unit, 66...transmission circuit, 67...signal acquisition unit, 68...power acquisition unit.

Claims (10)

In a lighting monitoring and control system in which the power lines that supply power to the airport lights also serve as a communication medium,
A transformer having a primary side connected to the power line and supplying power taken out from a secondary side to the lamp;
A slave station provided between the secondary side of the transformer and the lamp;
a master station that communicates with the slave station via the power line;
The master station includes:
a modulation unit that generates a modulated signal by modulating a carrier signal having a frequency band of 10 kHz or less with a multi-bit code using a modulation method that represents a symbol by a combination of a frequency shift and a phase shift;
an injection unit that injects the modulated signal into the power line in synchronization with a power supply cycle of the power;
The slave station,
a receiver for receiving the modulated signal from a power waveform on the secondary side of the transformer;
A demodulation unit that demodulates the received modulated signal to reproduce the code. The light monitoring and control system according to claim 1, wherein the modulation method is a modulation method in which a two-bit code is associated with four symbols represented by two mutually different frequencies and phases. the modulation unit modulates a plurality of carrier signals having different bands using the modulation method to generate a plurality of modulated signals having different bands;
The light monitoring and control system according to claim 1 , wherein the injector injects the plurality of modulated signals into the power line. a response signal generating unit, the slave station, generating a response signal by modulating a carrier signal with a response code to the demodulated code according to the modulation method;
The light monitoring and control system according to claim 1 , further comprising: a transmitting unit configured to transmit the response signal to the master station via the power line. the master station includes a control unit that repeatedly executes a transmission/reception sequence including transmission of the code from the master station to the slave station and transmission of the response code from the slave station to the master station;
The light monitoring control system according to claim 4 , wherein the slave station includes a synchronization processing unit that uses a first signal in the transmission/reception sequence as a reference signal and synchronizes with the reference signal. The light monitoring and control system according to claim 4, wherein the transmitter transmits the response signal by an overvoltage generated by shorting/opening the secondary side of the transformer in synchronization with the power supply cycle. The light monitoring and control system according to claim 4, wherein the transmission unit provides a guard interval before and after the signal section of the response signal. In a lighting monitoring and control system in which a power line that supplies power to airport lights also serves as a communication medium, a master station communicates with a slave station connected to the light via the power line,
a modulation unit that generates a modulated signal by modulating a carrier signal having a frequency band of 10 kHz or less with a multi-bit code using a modulation method that represents a symbol by a combination of a frequency shift and a phase shift;
and an injection unit for injecting the modulated signal into the power line in synchronization with a power supply cycle. A light monitoring and control system is used in which a power line that supplies power to airport lights also serves as a communication medium, and a slave station that can be connected to the light includes:
A receiving unit that receives a modulated signal generated by modulating a carrier signal of a frequency band of 10 kHz or less with a multi-bit code by a modulation method that represents a symbol by a combination of a frequency shift and a phase shift from a power line that supplies power to the lamp;
a demodulation unit that demodulates the received modulated signal to reproduce the code. A communication method for use in a light monitoring and control system in which a power line that supplies power to airport lights also serves as a communication medium, in which a slave station connected to the light communicates with a master station via the power line that supplies power to the light, comprising:
The master station modulates a carrier signal in a frequency band of 10 kHz or less with a multi-bit code to generate a modulated signal by a modulation method that represents a symbol by a combination of a frequency shift and a phase shift;
the master station injecting the modulated signal into the power line in synchronization with a power supply cycle;
the slave station receiving the modulated signal from the power line;
and the slave station demodulating the received modulated signal to recover the code.