PLC Scan Time for Modbus RTU with Generic Ethernet Module

Hello Experts,

Once again I am here as could not find information.

I am reading and writing to Modbus RTU device through Anybus communicator configured as generic ethernet module with PLC 1756-L71.

My code sends RTU frame by writing to Anybus output table and incrementing the trigger byte and then waits for the reply (trigger from the input table) before sending next transaction. The issue is it takes between 27ms to 57ms between output trigger being set and input trigger received.

I am not using cyclic rather using produce/consume in Anybus configuration.
PLC scan time is 2ms approx. Generic Ethernet module RPI is 5ms.

Any idea where this 27 to 57ms time comes from?

also where do you set 3.5 character time or would it even matter in my case as I wait for reply before sending next request?
Your help is appreciated.

I'm unaware of a configuration that would use "Produce/Consume" connections to an Anybus Communicator. That's for Logix-class controllers. You almost certainly have an ordinary cyclic connection.

Be sure that your Modbus command and the incrementing Trigger byte all get sent to the Output tag at the same time (I recommend a CPS instruction).

The Communicator probably waits for the Modbus device to reply over the serial connection before incrementing the Received Data tag.

How large a transaction ? What's the serial data rate on the Modbus RTU side ?

In very rough terms a serial byte takes 10/bps seconds to transmit, so a 20-byte packet at 9600 bps would take about 20 ms just to travel on the wire, without taking into account the response time of the slave device.

Hi Ken,

Produce/Consume (Generic Data Mode) is used within Anybus communicator to communicate with Modbus RTU node over serial network.

The transactions is 14bytes @ 19200bps.

Thanks for your reply.

If your timing information is coming from the 1756 I don't know that I'd call 27ms slow. Yes the scan time of the PLC is fast but there's entire transaction that takes place basically twice from execution of the 1756 asking for the data to the completed reply back to the 1756. The big missing factor is the response time capability of the Modbus RTU device. How long does it take to process the request and respond?

Originally posted by whussain6:

The transactions is 14bytes @ 19200bps.

Click to expand...

I assume you mean a 14-byte payload without the Modbus RTU wrapper around it.

As has been said, the real wildcard is the response time of the serial device being sent the packet. That is kind of an unknown at this point.

Keith