Anyone who is familiar with GE Fanuc controllers will recognize the two protocols mentioned in the title; SNP and SRTP.

In Proficy Machine Edition, it uses either of the two protocols to communicate with controllers and HMIs.

My dilemma is I want to be able to multidrop 16 PLCs over Modbus RTU and have a single programmer (Proficy ME) doing the monitoring/programming.

Naturally, this can be done with a SNP multidrop network but because of the 8 devices limitation of the SNP protocol we are seeking alternative solutions.

That is why we are thinking about Modbus. Since the software will only communicate with the controllers either by SNP or SRTP. Are there gateways/converters that can be used to achieve a SNP/SRTP connection to a Modbus network?

 

Depending what PLC CPU you are using you might not need a gateway. The newer GE PAC CPUs (Rx7i & Rx3i) can be Modbus TCP or RTU slaves. Making the CPU a Modbus master involves programming COMM_REQ functions and is a bit more involved. Firmware version are important to know because I had to update my CPU firmware to support Modbus TCP.

If your using a 90-70 or 90-30 CPU, I'm not sure but I don't think they support Modbus TCP. I think you need a card for RTU communications.

David

 

But when programming with Proficy Machine Edition; you have a choice between SNP (Series Ninty Protocol) or SRTP (Service Request Transport Protocol).

So how do I connect my mulitdropped Modbus controllers to Proficy ME?

 

Sorry, I must have misunderstood your original question. GE Proficy ME can only connect via SNP or SRTP.

 

Coming from a Siemens world... I have a question (or two). What is the difference between RTU and TCP modbus? How can I connect a Siemens PLC to a Telemecanique PLC via Modbus?

 

Hello;

If you need to use Modbus-TCP with a Siemens S7-300 or 400 CPU, you will need a CP343-1 or 443-1 (Ethernet comm. processor) and special modbus-TCP blocks. These are downloadable here:

http://support.automation.siemens.com/WW/view/en/22660304

If the connection is through Modbus-RTU or ASCII, then you need a serial processsor (CP341 or 441-2) and "loadable drivers" for Modbus master or Modbus slave (there are for sale) from your Siemens rep. Parametrisation software and example programs come with the loadable drivers package.

Hope this helps,
Daniel Chartier

 

http://213.130.34.122/pdf/en/applicom_GATEway.pdf

The above link is a gateway datasheet from Woodhead which supports multi TCP/IP messaging between different PLC manufacturers including SRTP, MODBUS/TCP. The P/N(APP-ESR-GTW) has a serial port which also support SNP.

Regards,

 

There is brief explanation about the differences between modbus RTU and modbus TCP at http://www.simplymodbus.ca/TCP.htm

 

Hi,
there are plenty of explanations about modbus, search the control.com site or http://www.modbus.org (they mirror posts from control.com that mention modbus) You can connect a Siemens PLC to a Telemechanique PLC using a simple serial lead, for example S7-200 to TWIDO. The serial ports of the Siemens can be
programmed as a modbus masters or slaves.

Marc Sinclair

 

just wondering...

is there any code for using modbus at the lowest level on microcontrolers?

consider for example one need to make a logger of the modbus data on an ethernet link at the master end.

that anyone now of such projects?

thanks in advance
Hugo