SLC 5/03 MODBUS slave

We are talking to a potential customer that has a test rig that the computer has died on. It is confusing to me what exactly they have, but this is what I think I know.

1. There is an SLC5/03 system with digital and analog ins and outs that controls part of the rig. This SLC system talks to the computer with MODBUS (atleast it is believed to be MODBUS).

2. The SLC system controls a vfd to control air flow. This is believed to be 4-20ma and digital i/o.

3. There is a second Allen Bradley plc (maybe a micrologix). The second PLC controls a coolant system.

4. The SLC system sends setpoints and process variable to the micrologix system via analog outputs.

5. In my reading, it appears that the SLC cannot be a modbus slave, it can only be a modbus master.

6. Our software can only be a modbus master, cannot be a modbus slave.

7. One option is that we replace the SLC system with IO that we control from our software. This would probably be the most straight forward, but don't know if potential client would go for it.

8. Another option that I have thought up is to put a Automation Direct Click PLC in line. Let the SLC be that master to the Click on port 2 (rs232) we could be the master to the click on port 3(rs485). I don't know if that would work or not.


We do not do PLC systems and have no prior knowledge of Allen Bradley PLCs. I personally have started to play with PLC as a personel play/learning experience.

Our system is a full data acquisition and control system running on Linux.

Any thoughts?

Thanks

You are correct that the SLC-5/0x controllers do not have Modbus RTU Slave functionality built-in, and Modbus RTU Master was a relatively recent addition to the operating system.

Follow the cables; look for a RS-232/RS-485 converter of some sort connected to that MicroLogix, and see what sort of cable connects the computer to the SLC-5/03.

The default protocol for the SLC-5/03 Channel 0 serial port is called "DF1 full duplex". There are a handful of serial protocol bridges on the market that can convert Modbus RTU to DF1.

The only protocol on the SLC-5/03 Channel 1 network port is called "Data Highway 485". This is also supported by the MicroLogix family, and may be how the MicroLogix and the SLC are communicating with one another.

If I were in your shoes, I would hire an integrator with some A-B experience to document the system and add a ProSoft Technology MVI46-MCM module as a Modbus RTU Slave. Do all of your communication to and from the controller via that module.

It's not cheap, but in my experience retaining a reliable control system (that can be serviced by the majority of controls integrators in North America) is worth a lot more in reduced labor and reduced risk than the cost of a little hardware.

Ken Roach said:

You are correct that the SLC-5/0x controllers do not have Modbus RTU Slave functionality built-in, and Modbus RTU Master was a relatively recent addition to the operating system.

Follow the cables; look for a RS-232/RS-485 converter of some sort connected to that MicroLogix, and see what sort of cable connects the computer to the SLC-5/03.

The default protocol for the SLC-5/03 Channel 0 serial port is called "DF1 full duplex". There are a handful of serial protocol bridges on the market that can convert Modbus RTU to DF1.

The only protocol on the SLC-5/03 Channel 1 network port is called "Data Highway 485". This is also supported by the MicroLogix family, and may be how the MicroLogix and the SLC are communicating with one another.

If I were in your shoes, I would hire an integrator with some A-B experience to document the system and add a ProSoft Technology MVI46-MCM module as a Modbus RTU Slave. Do all of your communication to and from the controller via that module.

It's not cheap, but in my experience retaining a reliable control system (that can be serviced by the majority of controls integrators in North America) is worth a lot more in reduced labor and reduced risk than the cost of a little hardware.

Click to expand...

We will probably have to go back to customer site, but from the crappy drawings we have it shows the SLC to Micrologix is analog outs from the SLC to analog ins to the Micrologix.
Am I correct in thinking that both DF1 and DH485 are Allen Bradley communications protocols?

Yes, DF1 and DH-485 are Allen-Bradley specific protocols.

DF1 is not entirely dissimilar to Modbus, but with a lot more command codes and looser timing requirements. Most non-AB devices that want to communicate with A-B controllers use it, and it's pretty well documented.

DH-485 is much more proprietary and the token-passing timing makes it very hard to implement except on a dedicated microcontroller. It's made for small serial networks (32 nodes, 19200 baud).

The backdoor method you might be able to use is if the MicroLogix controller has two serial ports (some models do). You might be able to run Modbus RTU Slave on one, and DF1 or DH485 on the other, turning that MicroLogix into a data gateway in place.

But my primary recommendation remains: Prosoft serial modules for the SLC platform are bulletproof and can be implemented by any A-B capable integrator.

There is a remote possibility you have one of these. See link.

http://www.modbus.org/viewdevice.php?id=223