PLC5/RSLogix5/1771-IFE/C help needed

[rta53]

Most of my lunch hour today has been spent searching the forum for help with this issue. While I did get some help from the various topics I still am not clear on some things.

First of all, I have not messed with a PLC5 in almost 15 years. We have primarily used SLCs on our systems. I would not be using a PLC5 at all except for the fact that the customer insisted on it.

Anyway here is my setup and questions.

12 Slot Chassis
1785-L30B Series E Processor
(2)1771-IFE/C Analog modules configured for single-ended current and located in slots 1 and 2 of the chassis.
I am using 1-slot addressing.
I also have 2 1771-IAD and 2 1771-OAD modules but I'm not concerned about them right now.

I allowed RSLogix5 to automatically create the data files for the BTR and BTW instructions and also to automatically create the ladder logic for the block transfers. So far so good. I think.

So here are my questions:

1. Do I have to do any additional configuration of the configuration words/bits other than what is done in the Setup Screen?

2. In the Raw Min and Raw Max fields do I enter 0 and 4095 for 4-20mA inputs?

3. What should my Data Format be? It defaults to BCD.

4. How do I get scaled data into my ladder logic since there is no SCP instruction.

Thanks,
Randy

 

[Ron Beaufort]

Greetings Randy,

1. Do I have to do any additional configuration of the configuration words/bits other than what is done in the Setup Screen?

normally, no ...

2. In the Raw Min and Raw Max fields do I enter 0 and 4095 for 4-20mA inputs?

usually ... especially if the inputs will be used for a PID instruction ... but see below ...

3. What should my Data Format be? It defaults to BCD.

set this for “Twos Complement Binary” ... BCD is for the much older PLC-2 platform ...

4. How do I get scaled data into my ladder logic since there is no SCP instruction.

normally we use math functions ... most people prefer a CPT ... tell us what values you want to see for “4” milliamps ... and what values you want to see for “20” milliamps and we’ll post the CPT code for you ... or click here

http://www.ind-info.com/training/y=mx+b_part1.pdf ... and here

http://www.ind-info.com/training/y=mx+b_part2.pdf if you want to “do it yourself” ...



or (back to the configuration screen) ... you can set the Min and Max scaling values for the values that you want ... example: Min of 32 will produce a “32” reading at 4 milliamps ... Max of 212 will produce a “212” reading at 20 milliamps ... but keep in mind that these will be INTEGERS with no fractional parts allowed ... so you can’t get “98.6” this way ...



final thought ... have you set the internal jumpers inside the module? ... don’t forget those!

 

[Ron Beaufort]

and one more thing ... make SURE that you’re actually using 1-slot addressing ... you should check this on the “Processor Status” screen ... NOT in the I/O Configuration feature ... see this post ...



if it turns out that it’s NOT 1-slot, then post again ... you’ll probably have to redo the Block Transfer rungs ...

 

[Ron Beaufort]

(2)1771-IFE/C Analog modules configured for single-ended current and located in slots 1 and 2 of the chassis.

and another thing ... are you sure that these aren’t actually located in slots 0 and 1 of the chassis? ... secret handshake: don’t count the processor’s (far left) slot as 0 in the PLC-5 system ... that works for SLC systems - but not for the PLC-5 platform ... so skip the processor’s slot (just don’t count it at all) and start with 0 for the next slot ...



going further ...

12 Slot Chassis

(this is picky but it’s more cause for future confusion) ... it might say 12-slots on the label ... but if you count them, you’ll probably find out that there are actually 13 ... we don’t normally count the processor’s slot on the far left ... ...



and if you haven’t run into it yet, be aware that the first 8 slots on the left (not counting the processor of course) will be addressed as “Rack 00” ... the next (last) 4 slots will be addressed as “Rack 01” ... that's assuming of course, that you're using the 1-slot addressing mode ...



and why are you using single-ended wiring? ... my best guess: probably because you need more than the 16 channels (8 per module) that you’d be able to use by going with differential ... that’s perfectly valid ... but if that is NOT your reason, then let’s talk about it ... differential wiring is the preferred method ... as long as you can get away with it ...



if you don’t mind posting your .RSP file, we’ll be glad to take a look at it for you ...

 

[rta53]

Ron,

As far as the scaling issue goes tell me if this would be ok. Say I want to end up with a range of 0 to 100.0. I could use 0 to 1000 as my raw data and then use a CPT instruction with a destination of F8:0 and Expression of N10:4*0.1 Would this work?

You are right about the modules being in Slots 0 and 1.

I am using single-ended inputs because I am doing 16 inputs per module. They are all 2-wire transmitters.

I have set the switches on the chassis backplane to 1-slot addressing. What else is there to do? I have not yet connected to the processor.

Randy

 

[Ken Moore]

As far as the scaling issue goes tell me if this would be ok. Say I want to end up with a range of 0 to 100.0. I could use 0 to 1000 as my raw data and then use a CPT instruction with a destination of F8:0 and Expression of N10:4*0.1 Would this work?

That would work, but you loose some resolution.

One method, not by any means the only method, is to use a CPT instruction.

Assume the raw counts are 0-4095 and stored in N10:4, and the scaled value will be 0-100.0 stored in F8:0

Then in a CPT enter the following
Destination: F8:0

Expression: (N10:4 | 4095) * 100


So if the raw counts were 1000, your scaled value would be: about 24.42

 

[Ron Beaufort]

As far as the scaling issue goes tell me if this would be ok. Say I want to end up with a range of 0 to 100.0. I could use 0 to 1000 as my raw data and then use a CPT instruction with a destination of F8:0 and Expression of N10:4*0.1 Would this work?

this should be fine ... for example: suppose that a 16.90mA signal comes in ... your module scaling should give you a reading of 806 at N10:4 ... multiply that by 0.1 and you’ve got 80.6 at F8:0 ... does this sound like what you need? ... going further: if this is ALL that you need, then I’d recommend just using a simple MUL instead of the CPT ... it will execute faster and take less memory (assuming that you care about things like that) ... on the other hand, it’s not nearly as glamorous as the CPT ... take your pick - dealer’s choice ...

You are right about the modules being in Slots 0 and 1.

sometimes I get lucky ...

I am using single-ended inputs because I am doing 16 inputs per module. They are all 2-wire transmitters.

should be ok ... if you start having “signal noise” issues, come back and see us ...

I have set the switches on the chassis backplane to 1-slot addressing. What else is there to do? I have not yet connected to the processor.

be sure to properly set the little jumper on the backplane ... it asks “Using a power supply module in this chassis?” ... if the power supply is mounted/bolted-on to the OUTside of the chassis, set this to the “N” position ... if you have a power supply that slides into a slot INside the chassis, put this jumper in the “Y” position ... the processor goes totally NUTS if you get this one wrong ...



how about the IFE’s internal jumpers? ... you didn’t mention those yet ...

and if the processor is connected to a Data Highway + network, you'll need to set the station/node address DIP switches on the processor itself ... the SLC uses software settings for this ... the PLC-5 uses DIP switches ...

PS EDIT ... and Ken's given you good advice on how to squeeze more resolution out of the module ...

 

[rta53]

how about the IFE’s internal jumpers? ... you didn’t mention those yet ...

Yes, I did set the jumpers.

I have used single ended inputs on SLCs without any problems. Are PLC5s more prone to noise? Seems to me that the PLC hardware manuals go a little overboard with all the grounding requirements.
I have always made it my practice to ground my shields at the terminal blocks and not at the module as the manual suggests.

Also I would normally put my digital modules in the first few slots of a chassis and then my analog. However the manual says to start with lower voltage (such as DC and analog) modules near the processor and then place the higher voltage to the right. I followed what the manual suggested, but I seem to remember seeing a picture of a chassis layout posted in another thread that had the analog modules on the far right. Does it really matter?

 

[Ron Beaufort]

I have used single ended inputs on SLCs without any problems. Are PLC5s more prone to noise?

no ... these things are pretty much bullet-proof ... the only reason that I asked is that sometimes people just use the single-ended setup because it’s the default-straight-out-of-the-box configuration ... you should be fine ...

I followed what the manual suggested, but I seem to remember seeing a picture of a chassis layout posted in another thread that had the analog modules on the far right. Does it really matter?

I’ve seen them plugged in ALL sorts of ways - and they work fine for years ... I wouldn’t worry about it ... but follow the book where possible ... why go out of your way looking for trouble? ...



let us know how things work out ... good luck with your project ...

 

[mellis]

Whether 'tis nobler to single end, or to press forth...

Ron,

I'm kind of surprised to hear you say "... differential wiring is the preferred method ... as long as you can get away with it ...". I'm curious about the experience behind that statement. The 1771-IFE manual says "To minimize ground-loop currents on input circuits: use single-ended mode whenever possible." (it goes on to list a few other considerations)

My personal experience is most of the installations I have seen using differential-mode have eight 2-wire transmitters all powered from the same supply. This means that half of the available channels on the IFE module are all tied together at the power supply common happily measuring 0vdc day in, day out. It just seems wasteful to me.

Sure, if you have some funky non-floating output 4-wire transmitters you may not be able to use single-ended mode directly. Or, if you have multiple instruments in the 4-20ma loop like recorders or indicators, that can also cause you a problem if you can't arrange the circuit so that the power supply common goes to the 1771-IFE for all loops. Fortunately, all these situations are becoming much less prevalent. The vast majority of new installations I'm involved in are simple "transmitter to input" without any complications.

If I run into a badly designed 4-wire transmitter that doesn't have a floating output, I prefer to use a signal isolator on that device. If I had a bunch of them, I'd consider using differential-mode. At some point, it makes economic sense but most of the time differential-mode is the expensive option.

I am interested to hear your experiences,

Mike Ellis

 

[Ron Beaufort]

Greetings Mike,



now you’ve got me wondering ... maybe (probably) this will be the “new-thing-that-I-learn-today” ...



the following is the way it was explained to me years ago (and I’ve never had a reason to doubt it - until your post) ... anyway ...



with differential wiring it’s easier to have two wires for each channel ... one wire carries the signal in from the transmitter ... the other wire carries the signal back (the return path) ...



with single-ended wiring the signals (up to sixteen of them on a 1771-IFE module) all SHARE (usually) only ONE single common line (the return path) ... so you (usually) have sixteen wires bringing the signals IN - but only ONE wire taking all sixteen signals back out ...



and so with single-ended wiring, the one common/return line carries the “algebraic sum” of ALL of the signals ... and so it’s easier (or so I’ve heard) for “cross-talk” and “noise” to couple from one channel to another with single-ended wiring ...



like I said, that’s the way it was explained to me back when I was too young to know any better ... now I’ll have to think it through ... I’ll get back to you ... it might be a week or so, but I’ll come back ...



going further ... it was also explained that there is a “trade-off” situation going on here ... specifically, why on earth would we ever willingly select DIFFERENTIAL wiring and limit ourselves to only eight channels per module? ... when we could just as easily select SINGLE-ENDED wiring and get sixteen channels from the same module? ... in other words, we could just ALWAYS choose single-ended and double our bang-for-the-bucks ... the downside of single-ended (the “trade-off” so to speak) is that “cross-talk” and “noise” issues are more apt to occur with single-ended due to the shared common/return path ... at least that’s the way it was explained to me ... now - after your post - I’ll have to think this through ...



and come to think of it, when this was explained to me, I was working for an Allen-Bradley distributor ... maybe ... just maybe, there was a “financial” incentive to recommend differential over single-ended ... eight channels compared to sixteen channels ... heck, we get to sell twice as many modules this way ... (now this would be a good place to insert a smiley-face but I just can’t bring myself to use those things) ...



as I said, let me think it through ... and in the meantime, of course, I’d appreciate any comments from anyone else out there ...



as for “experience” ... frankly I’ve never seen any clear-cut advantage to either method ... I’ve seen both methods work fine for years ... I’ve seen both methods give problems (rare indeed) ... and regardless of either method, the few problems that I have seen, have usually (ALWAYS as far as I can remember off the top of my head) been caused by poor cable-routing practices or loose connections, etc. ... and NOT by any inherent short-comings of either wiring method ... specifically, I’ve never heard of any real-world situation where a signal problem on a single-ended system could be cured by simply rewiring it as a differential system ... or vice versa ...

The 1771-IFE manual says "To minimize ground-loop currents on input circuits: use single-ended mode whenever possible." (it goes on to list a few other considerations)

yes, I found that reference ... I’m just wondering if possibly they’re drawing some distinction between problems with “ground-loop currents” as opposed to “cross-talk” and other types of “noise” problems ... but probably not ... let me think about it some more ... maybe run some lab experiments ... like I said, I’ll get back to you ... but thanks for bringing it up ...



and one final thought ... anyone reading this should keep in mind that the 1771-IFE modules that we’re discussing are not “isolated” modules ... specifically, even when using DIFFERENTIAL wiring, the “back sides” of all of the channels are still physically connected inside the module ... I’m going by memory here but I think that the 1771-IL module does provide truly isolated channels ...

 

[rta53]

Thanks for all the help so far. I seem to have my hardware issues mostly taken care of. I have successfully gone online with my processor using a PCMK card. I had some problems initially when I tried to set up a second DH+ driver in RSLinx. I got a blue screen and ended up having to go into the registry to delete the driver. All is well now.

Well I now have what seems like a really dumb question. When programming a SLC I use a simple JSR to jump to all my other program files. How do I do this in RSLogix5? The JSR looks like it is for jumping to certain portions of ladder logic and not just another program file.

 

[Ron Beaufort]

program the PLC-5 JSRs exactly the same way as you're used to with the SLC platform ... when the screen asks you for an "Input Parameter" just hit the Enter key ... when the screen asks you for an "Return Parameter" just hit the Enter key ... hitting the Enter key without giving an address (a "null" so to speak) will cancel that part of the instruction ...

secret handshake: the SLC can't "pass parameters" with its JSRs ... the PLC-5 CAN - but it doesn't need to ... without the extra "parameter passing" the PLC-5's JSRs will work just like the SLC's ...