Recently, we encountered problems communicating on the bus with I/O modules - the bus was disturbed and normal communication was practically impossible. The cause was one of the modules powered by a faulty transformer and thus about 6 V, so it was permanently reset and transmitted interference on the bus.
This can be recognized by the fact that its red Tx LED flashes permanently - in normal situations it flashes only when it answers a query from the controller, ie the modules gradually alternate in flashing.
Another thing is the correct bus end. The blue and white DIP switches on the modules are used for this purpose: switches 1 and 2 in the ON position (inside the module) terminate the bus.
The bus should have a line topology and end at the beginning (eg on M011, or on the MiniPLC at the respective interface) and at the end, ie on the last module in the series.
It is always a good idea to terminate the bus. With a smaller number of modules, it can happen that the communication will work without problems even when the bus is not completed; however, when adding more later, mysterious problems can occur - some modules sometimes disappear, the runtime reports CommErrory, etc.
If there are a large number of modules on the bus with the switches on, this can also cause problems. Therefore, in case of communication problems, it is advisable to first check all BUS END switches on the modules if the bus is terminated correctly.
It is also important to choose the right converter to RS485: some types (eg ICPCON 7561) do not have time to switch to listening mode in time after sending the query and do not have time to read the quick response of the module. It looks like the module is defective, but it works properly when connected to a MiniPLC or a controller with M011.
These problems are difficult to diagnose because they work with some types of modules and not with others (newer ones have a more powerful processor that responds with virtually no delay).
An artificial delay would significantly reduce the effective baud rate and is therefore not used. Therefore, use M011 converters that have switching fast enough.