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

It’s possible that the some of the S-Bus telegrams transferred from the PCD S-Bus master to the S-Bus slaves over RS485 are malformed, and the S-Bus slave does reject the request from the master.

This could lead to the situation that for example the PCD S-Bus master don’t receive actual values from E-Line RIO’s or that the program download of a PCD program from the PC to a Slave PCD which is located behind a gateway PCD, fails. 

The firmware update of the PCD who act as S-Bus master with a firmware 1.28.34 or newer solves the issue.

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.



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.



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.


PCD1 / _Firmware Classic

PCD2 / _Firmware Classic

PCD3 / _Firmware Classic

PCD1 / E-Line

Last update: 16.11.2018 15:47

First release: 16.11.2018 14:00

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