VersaMax Micro to Modbus Slave

[Joe_WaZoo]

Guru's,

I need a little help trying to get a GE VersaMax PLC to be a Modbus Slave. How do I read / write access into the GE Machine Edition? Then how do I write ladder code to interact with that port.

Thanks,
Joe_WaZoo

 

[Steve Bailey]

Presumbly you're talking about one of the serial ports? Just configure the port for Modbus RTU in hardware configuration. No ladder logic support required unless...

Modbus RTU can't access some the PLC memory. The most important inaccessible areas are internal discretes (%M memory) and analog outputs (%AQ memory). If your modbus master needs access to those values you'll need to add logic to copy them to areas that are accessible.

All GE memory areas start with address 1, not zero.
Modbus discrete inputs correspond to %I memory in the PLC.
Modbus discrete outputs correspond to %Q memory in the PLC
Modbus analog input registers correspond to %AI memory in the PLC
Modbus data registers correspond to %R memory on the PLC.

Modbus treats discrete inputs and analog inputs as read-only. GE imposes no such restriction, although any %I or %AI addresses assigned to modules will have the actual module data overwrite anything you may try write to them.

 

[Joe_WaZoo]

Steve,

Thanks for the info. So if I understand this correctly, the GE can only be a Modbus Slave. I need to have that GE as Modbus Slave address #2. How do I link these Modbus registers to ladder logic tags?

Thanks,
Joe_WaZoo

 

[Steve Bailey]

So if I understand this correctly, the GE can only be a Modbus Slave.

That depends on the model. As far as I know, the VersaMax Micro and Nano models are limited to RTU slave. The modular VersaMax CPUs are configured as RTU slave in hardware configuration, but can be reconfigured as master via ladder logic. You enter the RTU station address in hardware configuration.

VersaMax PLCs use traditional addressing. The PLC tags will have an address.
Modbus address 000000 (Discrete Out) corresponds to PLC address %Q0001
Modbus address 100000 (Discrete In) corresponds to PLC address %I0001
Modbus address 300000 (Analog In) corresponds to PLC address %AI001
Modbus address 400000 (Data Register) corresponds to PLC address %R0001

If your RTU master needs access to the VersaMax PLC's internal discretes, (%M memory), you will need to add ladder to copy the %M bits to %I or %Q memory.