Why the RS-485 S-Bus communication between the PCD master and the slave does fail sometime, if FW 1.28.20…1.28.33 is used?

FAQ #102026

Symptoms
Programmable PCD devices that act as S-BUS master on RS485 with PCD Firmware >= 1.28.20 and <= 1.28.33 get no answer from Slave devices on some of the S-Bus master requests, although S-BUS address, baud rate, polarity and line termination are ok.

 

Possible effects of the issue

Until now we have found that E-Line RIO communication seems to be more concerned of this problem than e.g. RS485 S-Bus data communication between CPU's.

In some of the cases the effect was, that it was not possible to write to the outputs of the E-Line RIO's or the change of values on the E-Line RIO Inputs was not transfered to the S-Bus master.

With the concerned Firmware it might be very difficult or impossible to download the user program over a gateway connection.

PCD Firmware 1.28.x for all programmable PCD types are concerned.

 

Reason

The reason of the issue is a error in the Firmware of the S-Bus master.
The problem on the Firmware is, that telegrams which contain DLE characters (B5 or C5) as last character of the telegram (CRC) where malformed because the last character is missing.

Since the CRC is calculated during runtime this malformed S-Bus request occurs depending of the content of the S-Bus request.

The (malformed) CRC is transfered with the S-Bus request from the master to the slave.

If now the slave receives the S-Bus request and the received CRC does not fit with the calculated CRC, then the S-Bus slave reject the S-Bus telegram.

 

Solution

In case you use the concerned Firmware on an installation with RS485 S-Bus Data communication, update the S-Bus master PCD to the newest available Firmware >= 1.28.34.