For a new project I need to select PLC that can be accessed over Modbus protocol (it needs to act as a Modbus slave). Some manufacturers' websites left me confused : they mention special Modbus communicators, while others specify that their PLCs can directly be connected to Modbus network.

Has anyone had any experience & what PLC were you using?

Thank you very much,
Niksa
Geolux.hr

I've had good luck with Square D, and AB PLC's. I've used Digi One's Digi One IAP box which can freely translate almost any protocol to any other. I've also run across a Prosoft Technologies box once with very little effort. In the AB world, at least one of their Micrologix PLC's (but I forget which one) actually uses Modbus as the native "remote I/O" interface instead of one of the AB protocols (as a master, not a slave). All of these were "master" setups where I was trying to use the protocol "as intended" (to access remote I/O) or where I was querying the PLC from a PC. Needless to say, Modicon PLC's (now sold under the Telemechanique name) can use Modbus as both a master (remote I/O) and slave (PC to PLC) interface. The protocol was originally Modicon's but it has become so widespread and so generic that it is now a public domain standard and no longer controlled by Modicon.

In addition, the majority of the Modbus PC software can freely masquerade as either one. Except for the initial connection, Modbus is "symmetrical" in nature which means that the master/slave terminology is mostly semantics. Generally slaves generate only replies and masters generate only queries.

Modicon products use the licensed, proprietary Modbus Plus protocol that allows for multiple masters. I suspect that Modbus RTU slaves can be addressed from a Modbus Plus master, but could not state that as a fact.

Those devices that claim Modbus RTU compatibility talk over a serial RS-232 (point to point only) or RS-485 for multidrop, multiple slaves).

I also heard that A-B's higher end Micrologix has native Modbus functionality, but am not sure if it is mastre or slave. If it is a Modbus master, it could also function as a slave, but that would be an engineering implementation, not necessarily a fact.
A-B's SLC and PLC5 used a 3rd party Rockwell partner company, Prosoft, for Modbus modules that plug into the backplane (there might be others, but I'm familiar with Prosoft).
Automation Direct's controllers have native Modbus RTU functionality. I think upgrading to Modbus TCP (over ethernet) is an option card, if I recall properly.

Dan

See link below:

http://www.controlmicrosystems.com/

Hi, Niksa:

ICP DAS embedded PLCs with Modbus firmware are programmable slave I/O units with Modbus protocol and suit master-slave or query-response distributed I/O applications. The controllers are pre-installed Modbus firmware, therefore the PC, HMI or PLC can use Modbus/TCP and Modbus/RTU protocol to communicate with I/O modules in the unit via the RS-232/485 or Ethernet interface. Besides the slave I/O functionality, it is also a C language based programmable controller for stand-alone applications.

The SDK which ICP DAS provides, allow the customization of rich functions for Modbus communication, such as Modbus/TCP master/slave, Modbus/RTU master/salve, Modbus/ASCII master, etc.

You can find out more at http://www.icpdas-usa.com/i_8431_mtcp.html

I hope this is what you were looking for!

Cordially,

Ms. Vimaris Grau
www.icpdas-usa.com

DanW,

Modbus Plus requires a Bridge Plus (BP85) to access Modbus serial devices. It's also not an open network like Modbus.

Dr. No,

Mitsubishi offers Modbus modules for the FX3U and Q Series (including Modbus/TCP ethernet on Q).

You can use Omron PLC(CS/CJ) with SCU-31V1 module with Modbus macro.

AB Micrologix talks Modbus natively in either direction (master or slave). PLC-5, CLX, Micrologix, and SLC can do it but you have to write the protocol code yourself. Micrologix can pretty trivially act as a master by using built-in MSG instructions. As for being a slave...I'm not sure. Haven't tried it before but I bet it's easy to do. AB's web site has sample code specifically for doing it with CLX. All that the Prosoft module does for you is to SOMEWHAT automate/simplify the process. Setting up their modules is complicated.

SoftPLC can go in either direction, natively.

What you will find pretty quickly is that Modbus is effectively the "lingua franca" (universal language) of virtually all PLC's. Almost every PLC can use it. The only real catch is that sometimes you need some sort of optional module or hardware support.