M580 - SIL Modbus Communication limitations?

[ArunrajPremraj]

Dear All, 

My customer has asked some questions related to M580 safety PLC communication blocks for which i need answers. the questions are as below, 

1) What is the Maximum no. of Parallel/simultaneous TCP/IP connections that SIL CPU supports as modbus client?

2) What is the Maximum no. of Outstanding modbus requests?

3 )What is Maximum Buffer size for keeping Pending modbus requests?

I would really appreciate if someone can answer any of this

[DanielWeber · Accepted solution]

Hello,
there is no specificity with Safety - SIL Modbus communication limitations. Limitations are for Modbus TCP/IP.
You should find all the answers in the M580 Hardware Reference Manual, page 28/29
Modicon M580, Hardware Reference Manual 

I hope it helps.

[BevanWeiss]

If you are talking about safe communications then you should reference the Safety Manual

https://www.se.com/au/en/download/document/QGH46982/

In this you would use

S_WR_ETH_MX / S_WR_ETH_MX2

S_RD_ETH_MX / S_RD_ETH_MX2

Which require unique ID fields for all safe communication on the network (where there could be the possibility of unintended cross-over of base communication 'channels').

This essentially means if you have 100 PLCs, each communicating to 3 PLCs, then you would have 100 x 3 x 2 (bidirectional) channels.. = 600.

The ID is an integer, so you're unlikely to run into practical limitations with this before you would start to hit network limitations.

Co-ordinating the ID fields to ensure no duplicates is generally a little trickier however.  So you'd want to allow for ranges for each PLC.  Perhaps allowing 10 or so 'slots', 5 in each direction.  So PLC1 would get IDs 10-19, PLC2 IDs 20-29.. etc etc.