Hi guys.

I was in Tanzanioa till a fortnight ago. There I did a job using Premium + SCP-114 as a Modbus(RS-485)master to pull in data from 12 VFDs. The communication was done using the READ_Var function block and the RS-485 bus worked perfectly provided all slaves were energised and online. The responses to writing/reading about 12 words of data from any slave was less than 1 sec.

However....., when as single drive was powered down, what I noticed was the communication times drastically deteriorated, sometimes taking as much as 15-20 seconds for the changed data (typically motor speed, current etc) to appear in the PLC.

I tried many things to improve the performance, such as blocking read on all slaves when it was needed to write a value in a slave, also I tried various scheduling times for sending the read_var to each slave.. I'll appreciate If any one of you goes through the scheme I've attached. To see it you'll need Unity Pro ver.4.0. It is a FBD diagram and can be seen to make sense easily. I'll welcome all comments.

A million thanks up front, and with best wishes for your success...

Raj S. Iyer

Communications to the slave devices si Synchronous so you have to timeout on the units that are not enrgised before requesting data from the next slave. If possible you can make the timeout shorter so that it occurs faster. The other thing you could do is have a sub function that tests for the device being availalbe and if it is not set a latch bit. if the bit is on you do not request data from that device if it is off you do.

This is what we do in communications drivers.

Fred Loveless
Senior application Engineer
Kepware Technologies
http://www.kepware.com