ABB DSRC 113 Output interface module

Using code 15, 512 coils may be set in one message. Assuming two messages of 500 coils are used, the transaction takes (9 + 500/8) + 8 bytes x 2 messages x 11 = 1,760 bits. At 9600 bits/second, this takes 183.3 ms, plus the turnaround delay. It is possible to write this data four or five times a second if both devices can process quickly. A faster data rate would also speed the communications link. The coils must however be configured in a contiguous block (without gaps). It is good practice to plan a Modbus map to include blocks of contiguous data with room for spares. Some devices are not configured to respond to multiple write messages. If necessary, splitting the Modbus communication across more than one port or more than one CI will improve data rates and response times.

Modbus Slave Timing and Throughput Rules 

If the Trusted System is Modbus Slave, then it has no control over the messages received from the Master. If the Trusted System is delaying more than expected between messages, it is possible that insufficient time is allowed for communications processing between the CI and the Processor. It is possible to allocate some time to background tasks like this by specifying a sleep period between scans in the system configuration. This is especially a problem with systems that have no native I/O, e.g. bridged systems, and therefore do not have communications processing time allocated.

In the system configuration processor

 parameters shown below, set the sleep period to approximately 32 ms longer than the ‘current’ scan time seen in the Toolset debugger window. This will provide some time on most scans for communications tasks, and will even out the scan time. It will also lengthen the scan time, so ensure that process safety times are not compromised and the maximum scan time and I/O module watchdog times are adjusted proportionately to the change in average scan time.

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