Modicon Modbus uantum 140 CPU

First time using Concept, and Modicon, second time using ModBus.

I am writing software in Concept, for a slave PLC to work with a multi PLC system.

I do not have access to the rest of the system.

My company is building a piece of the puzzle, to be added later to the whole.

In S7, I recall using a function block to set up the slave for ModBus communication and to order the memory locations for the master. The master would poll the slave, the slave would offer up the memory in an array of words / bytes, and all was well.

I expect ModBus in Modicon to be a bit easier.

Is there a function I need to include in the slave code to point to the memory location? Or does the master just pull it directly from memory?

I am reading about the MSTR instruction (Modicon Ladder Logic Block Library User Guide (840 USE 101 00))

I don't find any corresponding slave instruction. Is there one?

Thanks

On Unity processors, ModbusTCP slave just works. I suspect the same here to. Slave functionality is builtin to the firmware, so the registers are just read. Only comment, and you may have seen this - is that on some of the processors (only ever used Unity, assume same is true here) you can choose how many registers to expose, so it is possible for a master to request registers that do not exist. A simple configuration change will fix that, though.

As above, i have only used Unity Quantums but we didnt have to do anything to get it working as a slave with a SCADA top end, just pointed the SCADA system at the registers we needed and it worked.

So, to summarise, i dont hink you need to do anything.

You mention the MSTR block. I assume then, you are using the 984 Ladder Logic editor?

What protocol are you actually using?
Modbus Plus in any of its versions, uses the MSTR function block.
Modbus is a serial communications and it will use the XMIT block.

But in either case there is no setup required in the remote devices, as long as you are only accessing the 4x registers.

Mr. Dungar,

Seems that XMIT is required for Modbus RTU out of the 984 PLC. And using MSTR would not communicate to slave device serially connected from the 984 port to slave device's serial port?

I'm new to (old) Modicon Quantum PLC and have seen various boards/threads about using MSTR or (purchasing) XMIT. I'd like some clarification.

984 113/3 Quantum PLC (Master by default) --> Serial Modbus RTU --> Micrologix PLC (Slave, node 2)

Also, seems MSTR only reads and writes to 4XXXX registers. No support for 0XXX, 1XXXX, or 3XXXX? Only codes are read and write and the documentation lists 4XXXX for top and middle entries/parameters of MSTR instruction.