The status byte RS485ErrorCnt controls the communication with the I/O modules. This is interesting for you if you want to integrate your own devices into your system.

The RevPi I/O modules exchange data with the RevPi device via a serial line. The data is transmitted electrically according to the RS485 standard.

The piControl driver scans which modules are connected during startup and reset. It then communicates cyclically with the modules in the order in which they are mounted on the DIN rail from left to right next to the RevPi. First, piControl reads the output values for the respective module from the process image and sends them to the I/O module. In response, the RevPi device receives the input values and writes them into the process image. This procedure is repeated with the next module and so on in turn.

Although RS485 is insensitive to interference, it can still happen that the data is disturbed during transmission between RevPi device and I/O module. piControl has an internal error counter for each module. If a transmission error has occurred, the counter is incremented. The maximum value of the counter is 255. As soon as a telegram has been transferred correctly between RevPi device to I/O module and back, the counter is reset to 0. The first error is ignored, i.e. only if two or more errors occur in succession the following error handling will become active.

The value RS485ErrorCnt contains the sum of all errors. The value can therefore be between 0 and 255*n if n I/O modules are connected. At the end of each cycle, the error counter is compared with the two limit values RS485ErrorLimit1 and RS485ErrorLimit2. If it has reached RS485ErrorLimit1, a message is output in kern.log. In the next version of piControl, the default values defined in PiCtory are written to the process image. If the error counter has reached RS485ErrorLimit2, the PiBridge communication is stopped.

If one of the values is set to 0, the respective check is deactivated.

RS485ErrorLimit1 is set to 10 and RS485ErrorLimit2 is set to 1000 as default.