CompactLogix - opinions on...

Hello

I would like to implement an automatic weighing system, and I´ve been thinking of using RS CompactLogix products. (SLCs and Micrologix are not options.. high prices!), so I would like to get some advice on this platform.

Is it recommended for small control applications involving serial communications to third party devices?, does the CPU have a Math co-processor?

Please share your comments and experiences regarding this controller.

Thank you !!

Make sure you read the selection guide COMPLETELY. The ConpactLogix has some gotchas about the rack configuration (limitations on how far certain cards can be from the power supply, available process memory (DeviceNet counts), etc). Especially read all the fine print on the L20. Our quoting group speced that one on a job and we ended up having to go to the L35E, which was a MAJOR increase in price. Also read the analog module fine print. Their response time can be terrible if you active all 4 channels (depending on what you are doing).

I do not think there is a math co-processor. There may be one in the newer, higher end versions that have come out (L32E and above). But even the lower end units have plenty of computation power (much more than a SLC).

The ASCII control is not all that great. I believe that Pro-Soft makes an MVS module for the Compact rack, but that will add quite a bit of expense to your project.

Hardy Instruments (www.hardyinst.com) has a weigh scale module that goes directly in the CompactLogix rack. It works well, provided you are not looking for "digital" weigh cells. The unit for the Compact can only use the more traditional analog weigh cells.

Darren

I like the CompactLogix L30 and L31 controllers for serial 3rd party communications better than all the other A-B controllers I work with. The serial port control and the array and string handling is the best in the A-B line.

I doubt it will be less expensive than MicroLogix, though. It's really not a comparison; CompactLogix has far more speed, memory, and instruction set power than the Micros do.

Most weighing systems I end up connecting to are using DeviceNet for digital communications, so that's an easy fit into the CompactLogix.

sherman4 said:

SLCs and Micrologix are not options.. high prices!

Click to expand...

SLC is similar priced or cheaper than CompactLogix.
MicroLogix is cheaper than CompactLogix.
For a small weighing application with analog inputs, a ML1200 or a ML1500 will be sufficient.

edit: Should have read your post more carefully. The weighing is done via a 3rd party device that you want to connect to serially (I think from reading your post). An SLC or a micro can still do this.

If your company is allready using a particular PLC, then you should not leave this unless there is a very good reason for it.
But if you are in the position where you freely can select the PLC, then I suggest that you try to imagine all the projects that you and your company may have in the forseeable future, and then select the PLC that scales well from the smallest project to the the largest. In that way you will avoid ending up with multiple platforms.

Sherman,
I would check out the scale you plan on using, most have Modbus communications built right in them without having to buy a special devicenet interface. The ML1500 from AB has a Modbus driver built into it as part of it's firmware. Using devicenet is fine but it is usually an adder for the scale and it is certainly an adder for the price of the PLC (pricey for what you get). ML1500 are pretty inexpensive.
You may also want to investigate how easy it may be to obtain data through the serial port using Ascii. Every scale I have interfaced to has an ascii serial port.
Now the cheapest and easiest is to use an analog output on the scale and an analog input on the PLC. Don't worry about analog response time, people are used to waiting for the scale to settle whether it needs to or not. Some simple discrete signaling (digital I/O) is also usually available for handshaking/resetting/latching.