PLC 5 Scaling Analogues

[Conor 4]

Hi, I was wondering if someone can help me out. I want to scale an Analogue input in PLC 5. If I was using an SLC , I could use the instruction SCP. So if 4 mA is -100 mBar and 20 mA is 100 mBar I could in the SLC set this up. What way do I do this in PLC 5. Thanks, Conor

[BobLfoot 302]

This is basic math which can be done with a CPT instruction. Something like ([{(READING - 4) / 16} *200] - 100) should work.

[Conor 4]

Thanks Bob, I know about using Expressions, but I thought that there might be something in PLC 5 that did the same thing as the SCP instruction, instead of doing an expression

[noxcuses 0]

I did a HVAC program once in a PLC5 that had quite a bit of analog inputs. I created a ladder file that used input parameters to do the math, then used a JSR with inputs to function somewhat like the SCP instruction. I wouldn't advise doing it that way if you are doing basic scaling with no filtering because it will add to scan time. IMO, the most efficient way in a PLC5 to do basic scaling is using a CPT.

[PLCMentor.com 60]

No such luck. The PLC5 is pre-SCP. Not sure why they didnt add it with the later versions of the PLC5's, but they didnt. But on the bright side, just think of all the fun you will get to have with math instructions! That and working with block transfers should allow for hours of fun. Seriously, the PLC5 methodology looks pretty dated with all the cool stuff we get in PLC's now but it's a rock. The processor and the I/O can be abused and take a beating without missing a beat.

[TConnolly 13]

Which analog input module are you using? Some of the 1771 analog input modules perform the scaling in the module.

[Conor 4]

All of my PLC5's have DeviceNet scanner cards and they do there I/O through DeviceNet. As Russell said, I have great fun with Expressions!!!

[paulengr 44]

You HAD to have the SCP instruction on a SLC because there was no scaling in the IO. In contrast, the PLC-5 didn't "need" it because those functions are built into the IO.

[Conor 4]

[Mickey 71]

That depends on the analog card you have and what you chose for its data format. For example for a 1746-NI8 module if you select "engineering units" for its data format you would get 4ma =4000 12ma =12000 20ma =20000 see picture below Edited 28 Feb 2010 by Mickey

[bigR 0]

Please know first what model of analog module you use. After knowing that download corresponding model manual then use guide as shown. Somebody use Block Transfer instruction then input all parameters as stated in the manual of corresponding analog. On new version RS5 software no SCP, but you use only block transfer instruction if the analog module is in the same rack. Edited 1 Mar 2010 by bigR

[Conor 4]

It was a 1746-NI8 card on an SLC. I saw exactly what you were saying Mickey, when I went into the Configuration off-line. You are then able to configure the card as suits. Thanks again, Conor.

[paulengr 44]

As you can no doubt tell by the literature for the SLC, it was originally intended as a sort of "low end PLC-5 system. One of the compromises was to eliminate the scaling features in the IO cards and just provide direct raw inputs to the PLC. As a compromise, they added the SCP instruction to do it in the processor. OK, now here's where it gets confusing though. That was all before RS-Logix 5/500 came along. At one time, even on a PLC-5, IO card "face plates" didn't exist. The reason that you see a milliamp display on your IO card in Logix 5/500 is simply that the software is providing that for you. Notice that there are no floating point values coming out of the IO card giving you a direct reading in milliamps. In fact, it's even more tricky than this. As PLC-5 users know, the IO list (not the same as the scan list) in RS-Logix 5 is entirely optional. The PLC itself doesn't care or need to know which IO card you are dealing with. The actual BTR/BTW commands don't have an "IO module" parameter at all, and once in a while you can accidentally "lose" this information and your block transfer instructions will revert to "generic" for a module type. This is merely convenience for you. Not so with RS-Logix 500 and the SLC processor which MUST have an accurate IO module list or it faults on you. You will notice the same trend with the newer families. The Micrologix processors do not have any sort of scaling in the IO cards. It's even more difficult with some of them which lack really any easy way to scale at all. On the ControlLogix/CompactLogix series however, they have scaling and lots of other niceties in their IO modules, far better than the PLC-5 world.