GE Versamax PLC Modbus communication

Hi All,

I am working with GE Versamax Micro 20 PLC on a project to read/write data from 2 Modbus slave devices. I have to read 3 different registers(420210,416600,416540) from Slave 1 and write 1 register(420210) to Slave1, also need to read 14 registers(adjacent registers) from Slave2 and write 28 registers(adjacent registers) to slave2. Since the address of Slave1 are not adjacent registers, Im using 3 COMREQ Function blocks for reading the data & 1 for writing the data. But registers of Slave2 are adjacent, so Im using 2 COMREQ functions 1 for reading and 1 for writing. Now the Modbus Reading/Writing takes too much time approximately more than 15 seconds to read/write a register. Can anyone help me to solve this issue??

Best regards,
Irshad

I would look at increasing the baud rate of the network to the highest common rate that all devices can be set.
Next, I would change the read registers in the PLC. Use some move commands to group the registers together, so they can be read in one command.

Hope this helps you out.
Regards,

Hi Garry,
The Baud rate is at its maximum. Versamax PLC supports maximum baud rate of 38400.
The memories of Slave 1 can't be changed, because Slave 1 is a VFD(Danfoss AQUA).
Slave 2 is ok, I changed to adjacent registers and using 2 COMREQ I can read & write data from/to slave 2

If you can't change the addresses you need to read, you're stuck with needing to perform six separate transactions. Three reads and one write with slave #1 and one read and one write with slave #2. If you are currently using timers to sequence the six COMREQ instructions that control the transactions, look into monitoring the status word of each COMREQ instructions and using it to trigger the next COMREQ.
Also, consider not executing the writes as frequently as the reads, especially if the data you are writing doesn't change very often.
Edit: I have no experience with that particular drive, but I have worked with several devices that support Modbus which allow you to customize an area of consecutive memory with whatever parameters you want. Check to see if Danfoss offers that option.

Dear Steve,

Thanks for your reply. Im doing the same, using a timer Im executing each COMREQ separately.

http://fiona.dmcs.pl/~rkotas/VersaMax%20MicroPLC%20Manual%20GFK1645J.pdf

The only other thing that I can think of is to add another port and have two masters on your PLC. Each taking to a separate slave.
It looks like the Micro 20 has that option.

Just a thought.
Regards,

Hi Garry,

Im using IC200USB002 communication board for modbus port. And the next port is programming port. So, which port you are talking about? If it is Programming port, is it possible to make the port as modbus master? Because by default it is configured as SNP slave. Im afraid to change the configuration of programming port, because if anything happened I can't communicate with the plc right?

Post your project file. I'll take a look and see if I can offer suggestions.

Depending on the firmware version of the Fc202 it supports function code 23 (17h) read/write multiple registers. Using that you only need one call for each slave.

Michal_dk said:

Depending on the firmware version of the Fc202 it supports function code 23 (17h) read/write multiple registers. Using that you only need one call for each slave.

Click to expand...

You'll be very very lucky to find a master that can execute FC 23.
Yes, FC 23 is in the Modbus spec, but I'm struggling to remember if I've ever run across a master that implemented it.

I've only used Modbus with Modscan32 (and Comm Suite) and here it is supported.

I'm wondering why it has been implemented then, if no controllers can use it.

Ah, the mysteries of life ;-)