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Can we read the data of a PLC over Modbus when it is in STOP mode?

Every PLC I have worked on, and that is a bunch, you can read and write data regardless of whether the PLC itself is executing the program or not.

Thanks for the reply.

I will modify my question, does the PLC gives the Current values of the Special Function Blocks, when read over Modbus in Stop Mode.

For example, if i m using a Up counter, and the count has reached up to 10, and now if I stop my PLC and restart it, the count will start from 0. Now in this case, during stop mode, if I will read the counter over Modbus, what should I get 0 or 10.

good point. I was not thinking of such things, because I have never
run across them. I don't recall if the prosoft modules continue to
function if the PLC is not in run.

--
Bob

If siemens plc power is on and i connect plc to pc through RS232 cable, can i read the data without having any problem?

In most cases, yes you can, especially if you're using a protocol the PLC supports natively.

I did have one project where I had to run a Festo CoDeSys-based PLC as a Modbus/TCP slave via its internal Ethernet port. It's possible, but the support isn't in firmware, it's a little software module Festo gives you to run with your application code, so in that case it talks only in run mode (to the Modbus master anyway). Takes a hell of a long time to boot too. Like 10 seconds or so after the application of power before it was responsive to Modbus polls.

If the PLC is using a communications processor (cp) for Modbus, you can not read data in the STOP mode; the cp is not running, or the data provided to it is no longer 'live'. With a Siemens Step7 plc using Profibus or Ethernet, all input modules that do not require a function block are still 'live' and available to something like a Wonderware I/O or Data Access Server (DAS). The Modbus module is served by read and write function blocks that do not run if the cpu is not in RUN.

I monitor a watchdog timer to verify that the PLC is in RUN mode, but Wonderware can continue to record the data from an analog input with the PLC in the STOP mode.