This problem occurred very recently. Our DCS system installed is the old Process Manager system from Honeywell. Our ESD systems are the Triconex TMR WITH THE 3006 PROCESSOR revision C09, and Triconex TMR WITH THE 3008 PROCESSOR. One fine evening, the communication between the two failed.

We have Modbus communication between the two and the DCS in a daisy chain configuration. After failure, the scan rate came up to 21 seconds. Our exercises led us to the culprit being 2 converters (232 to 485) which were promptly changed. Could anyone suggest a better communication scheme and point out the obvious faults in the current scheme ?

 

See www.national.com/an/AN/AN-1057.pdf which is titled "10 Ways to Bullet Proof your RS485".

 

I think the only solution that helps with failing hardware is to create redundancy. With modbus you probably have to install a second serial link and create software in the PM to select the signals you want to use. This would also be a solution if other hardware in the loop fails (e.g. serial interface FTA, IOP, ...). However, it is expensive and I would not do that if you can live with a temporary loss of view every so many years.