You don't mention whether this is Modbus TCp over ethernet or Modbus RTU over RS-485.
RS-485 is multi-drop, meaning one starts at the PLC, wires to the first device, the wiring continues on to the 2nd device, which then continues on to the 3rd device, and so on. Some people call this daisy-chaining.
So the entire length might be considerably longer than 50m if all the devices are not in a straight line.
My experience shows that the longer the total wiring length, the slower the effective baud rate. I've used 38K successfully within a control panel. But 38K wouildn't work all the time at 300m and I had to step down to 19K baud. Once I was limited to 9600 baud because the slave wouldn't run any faster.
I think 19K is a reasonable rate that can be implemented at moderate wiring runs.
Although you're sending or receiving 3 data bytes, the Modbus data word is approx 3x longer:
Request Message (Read (Gain 1) = ID Tag 001)
02 14 07 06 00 03 00 01 00 02 (CRC16)
Where:
02 = Address
14 = Function Code 20 (14 hex)
07 = Byte Count
06 = Reference Type
00,03 = File Number (Access Data Value)
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00,01 = Register Address (Standard Access Gain 1 - Tag ID #1) [/font]
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00 02 = Register Count (Floating Point Data) (CRC16)[/font]
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So, at 10 bytes per message, 11 bits per word (start, 8 bits, parity, stop) you have 110 bits per modbus request and about the same for a resposne.
So a single poll-response at zero turn-around time would take 220/19200 = 12mS. That might sound fast or slow, depending on how old you are (young ethernet guys croak. We who have used 300 baud go Wow!)
But slaves don't respond instantly. If a slave doesn't respond, the master waits until the its wait timer expires. That adds to the cycle time.
The master is not always ready to immediately send another request as soon as the reponse from the former request is received.
It the master gets an error message, it has to process it.
All these things add up to not really knowing what the throughput rate is until you wire it and try it. Unitronics might be able to tell you from an experience that someone, somewhere did X devices at 300mS, but that probably isn't the same configuration that you have.
Maybe someone else on the forum has Unitronics experience and can comment.
Dan
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