Hi Team,
Can you please have a look at the following customer inqury?
In one of our devices we have internally a RS485 bus with 12 nodes. In all participants we have the differential bus transceiver 75A176 in use. The bus runs stable, now we get messages from the field that some bus participants (always the same participants at the end of the bus) do not log on to the system or sporadically fail during operation. The error is difficult to reproduce even with devices that have this problem in the field, it is in the laboratory only occasionally succeeded in seeing this error in operation. I have dealt with the problem for some time and was able to find a participant as a possible cause. Here the wiring with the 75A176 of this participant, whereby the remaining participants have the same bus wiring:
Our RS485 bus is terminated on both sides with 120Ohm and switched with bias resistors against GND and +5V (failsafe biasing) as in the example below:
I can provoke the error by soldering a 100Ohm resistor directly across the signal lines instead of C4 at one station, then the stations behind it are mute or do not report at the bus. This would correspond to the error pattern in the field. This condition would occur, if the ports A and B at the bus device would be internally short-circuited or at the same time on LOW or HIGH, then I would have with the resistors R22 and R25 also approx. 100Ohm. According to the datasheet this state does not exist, but could it be possible to get the ports A and B in this (forbidden) state, which is then also latched, e.g. by disturbances from outside or via the supply voltage Vcc ? Are you aware of similar errors with this bus module ? Is there anything conspicuous about our circuit around the 75A176?
Thank you,
Franz
