Comparing ControlLogix to other AB PLCs


Thread Starter


Out of touch with A-B PLC world. Have done a lot in the past with 2, 3, 5, and SLC's but what's different about the ControlLogix series (in a nutshell). May be replacing a 2 with a ControlLogix.

Lee Drendall

Difficult to do in 25 words or less - I'd need a big nutshell. ControlLogix as a processor IS the next generation, and A-B is eager to phase out the PLC-5 series ASAP. You need RSLogix5000 (which only runs on NT or Win2000) to program it.

Major differences include no numeric addresses - all done by symbols, which reside IN the processor. Instruction set is similar to PLC-5. Based on a 32-bit architecture, so there is a double integer data type, used often for I/O tags. Curiously no string or ascii data type. User-defined Data Types (basically structures) are available, as are 2D and 3D arrays. Memory not as precious as in traditional PLCs.

A major "gotcha" right now is that the programming software and firmware versions must match, and the S/W at present is NOT backwards compatible. We are told this will go away with the next big revision (but that may be a while). Firmware is flash-upgradable, which can be a nightmare in a regulated industry.

Only serial comm on the processor itself. You add communications cards to suit. ControlNet being pushed to replace both DH+ and RIO, although both of the latter are available (on one card) as is Ethernet and DeviceNet and I think Fieldbus.

That barely scratches the surface, but hope it helps.
Logixs are easy/fast to program.
No memory allocation required (provided use OPC for MMI comms - somewhat slow with bugs!).
Create user defined data structures eg for valves/pumps etc and map your real I/O to these structures - any I/O changes then don't affect code/MMI.
Analog scaling is done on the cards - get meaningful engineering values in the PLC.
And unlike Siemens or Concept you can program Ladder online! (Better one well implemented programming language than a dozen crap ones!?).

Program versions are a pain.
Can't go online if PLC and offline copy get out of sync.
Can upload and merge (usually..).

Let's not forget that ControlLogix has no slot-dependencies like PLC5 or SLC. Your ControlLogix processor can sit in any slot of the chassis. Also, you can put as many processors in a chassis that the power supply can handle. Great for segmenting processes. ControlNet, DeviceNet, EtherNet, RIO, etc can communicate seamlessly in the same chassis across one common backplane. All I/O modules can be removed and inserted while under power (RIUP) without risk of damaging the module. RSLogix5000 programming software supports ladder logic design or function block diagraming if you like. There's a lot more!!
ControlLogix version 8.02 has just implemented the STRING type, with built-in manipulation functions. STRING is essentially a structure with a .LEN element (DINT) and a .DATA[] element (an array of 82 SINTs).

Question: Does it support indirect addressing on a bit? I.e. Vreeblefetzer.[this_bit] ?
I am still writing offline and have not had a chance to try the code in a live processor yet.
A few things that haven't been mentioned ... ControlLogix has a tasking model that allows periodic tasks as well as a single continuous task, so you can segment your slow logic like loops into a periodic task rather than the silly timers.
ControlLogix supports function block programming with a pretty good set of process blocks. I've converted a DCS to it with no problems.
The analog IO scales on the card and is configured in ControlLogix. No more code crowded with 0-4095 scaling.
You can connect multiple ethernet, devicenet and controlnet networks. The IO configuration can be pretty complex, with multiple devicenet segments distributed across a controlnet network.
The same programming software also works with their soft PLC, FlexLogix and CompactLogix platforms, giving a lot of flexibility.
And it bears repeating that the tag database and documentation is in the controller.




I want to now how many I/O real can we connect to single controller of control Logix.

Is really it is possible to configure scan time on each tag?

please help me out for this questions.

Thanking you.

Ramesh B Parmar

[email protected]
> I want to now how many I/O real can we connect to single controller of control Logix. <

The total number of IOs that can be connected to the single ControlLogix processor is 128000, this is from AB website. Out of these maximum 4000 can be analog IOs.

> Is really it is possible to configure scan time on each tag? <

You can configure the scan time for the loops.
For example, you have few close loops which need to have a scan time of 50 MS, in that case, you can configure them in a single periodic task (task = group of control programs) & assign 50 MS scan time for that program.