Should I keep Modbus or switch to analog control of old Siemens Simoreg DC Drives (200HP)?


Thread Starter

Paul Cunningham

We are preparing to install a used fabric processing line including seven large DC drive systems. It has a Modicon 984 PLC, three PanelMates, and the seven Siemens drives on a Modbus network. The PLC will be replaced with, you guessed it, ControlLogix. The debate is currently, should we abandon modbus in favor of harwired control? I have the docs for the drives and they are loaded with features. Our plant uses A/B, and Reliance products. We are comfortable with RIO, DH+, and are learning ControlNet. If we keep Modbus, which none of us have ever worked with, we don't have to fool with the panelmate comm. changeover, and may get a lot of valuable information to and from the drives, once we pass the learning curve. If we switch to harwired control, we know we can commision a functional machine, but will have to change the PanelMates comm. (easy, but expensive), add two analog output cards, (also expensive) and end up with very little feedback from the drives. Can CLogix do Modbus? Where can I get manuals for the modbus interface on these old Siemens Simoreg DC Drives? I've been to their website and found hardware and setup manuals, but nothing on the comm. cards. Oh by the way, our controls upgrade budget is about $20k! It barely covers hardware costs... That means we get to do all the engineering in house with our own talents and no hired help! I love my Job.

[email protected]
The easiest way to do Modbus in an AB PLC is with a module from Prosoft. The ones for the SLC and PLC are essentially ASCII BASIC cards with special firmware. I'm pretty sure there is one for the CLX but I don't know if it's the same type deal or designed from scratch. In any case, they're easy to use (just write data blocks to the card memory range) and will set you back about $1200. If you have the Modicon program and can't get the Siemens drive register mapping, you may be able to see what registers in the drives are being communicated with and how they're interpreted in the PLC. Kinda backwards I know, but better than nothing.
Steve Myres, PE
Automation Solutions
(480) 813-1145

Whats the model number on these drives? It might be easier to go with profibus and forget the modbus.
With only one PLC, three PanelMates and seven drives, I'm almost certain that that your "Modbus network" is really Modbus Plus. Modbus and Modbus Plus are very different. A Modicon PLC with MB+ will have a model number ending in 5, such as 984-385 or 984-785. I don't know about the Siemens drives, but I had a project with Robicon drives which had a daughterboard with an ISA slot for a Modicon SA85 MB+ network adapter card. PanelMates also used an option card for MB+ communications. If you have the 984 program, look for MSTR (Modbus Plus Master) function blocks. I expect a multiple of seven to read from and write to the drives. No programming is required for the PanelMates, since they handle the read/write to the PLC. Fortunately, ProSoft Technology does have a ControlLogix MBP module, but you'll have to go through all of the PLC and the PanelMate programming to figure out what coil and register addresses are used.

Good Luck,

Bob Peterson

Clgx can indeed do Modbus. Its a little clunky but it works well in my experience. You would need to get a Prosoft Modbus module. They are an AB partner of some sort and it just plugs right into a clgx rack.

There are two versions, the original and a faster version. What kind of fabric system is this (just curious)? Seven drives sounds like a slasher.