SLC 5/03 programming oddity mystery

Hello,

I've taken on yet another basket case old machine running an 7 slot SLC 5/03 PLC....

I have run into a situation where I see the same 4 rungs of logic repeated 4 times through out the main ladder. all of the addresses are identical among the 4 instances of the 4 rungs, and the fourth rung is a jsr to a ladder with basically 1 dummy rung and the other being a NC/NO of the same address thereby causing that run to always execute false.

What i cant understand is that your are seemingly latching the same addresses, calling the same ladder 3, and using the same t4:10 timer 4 times in the same ladder.

This looks intentional, as I am lucky to have a printout of the original program. the hard copy matches the current program fairly well, with some code added here and there, but the specific rungs in question are nearly identical. Id be forever grateful if minds more intelligent than mine would be willing to look at this little gem and shed some light on how they are utilizing the code to control a couple of outputs.

Specific addresses:

Searching B9/10 will take you to the 4 sections of logic, bit is used as a marker

What i am specifically going after is how O:6/2 can be expected to operate given the two instances of the OTE

There is one major difference that could possible have a big impact:

In ladder 3, rung 1, the original code only has in series NC O:6/2 and NO O:6/2 where the current code has the same NC+NO but addressed O:6/7 for both. Was the addresses incorrectly modified? Also in the same ladder, O:4/7 on rung 3 was added in at some point, the original program just has the original instance of O4:/7 in the main ladder 2.

The machine runs pretty well, but when it gets out of sequence, operators just mash buttons and hope for the best. It is a palletizing machine that makes a layer of cases then stacks 4 rows high, with 2 different case sizes, so basically only 2 recipes.

as a bonus, if anyone can tell me what sort of data liner display was intended to be installed with the machine, that'd be cool. Right now i'm trying to develop an idea for a modern touch screen to use the original alarming and messaging. Build date on BOM is 1995!

OMG an Alvey palletizer! You have my sympathies. I have inherited a few of those machines. I usually just delete everything except the patters and write my own code. I don't know what kind of drugs their programmers were on in the 1990s but their code was the worst I have ever seen from an OEM.


I learned from an Alvey field tech that the SLC program is just converted from PLC2. And the PLC2 was converted from relay logic.


The data liners I have seen on them before use discrete outputs to trigger messages, but I don't see outputs in the program like that.

oh that's funny. Kind of sad but funny. Yeah, the messaging code looks like they went through some integer file manipulation. I will probably just piggy back some internal bits and kind of bit bang the messages that seem needed the most.

I've seen cases where a machine was first conceived in a Siemens environment and converted to AB, so that has its own nuance to get used to, but I haven't seen a case in SLC where same outputs addresses and timers were used like they did here. AFAIK, that concept is a big no-no, so either they did it deliberate, or.....as your comment indicates, they messed up from day one.

I agree with JaxGTO

This is the time I would look over the full operation of the machine & write a completely new program.

As far as O:6/2 the JSR's are only called when that output is turned on in ladder 2. If JSR 3 is never called it will have no effect on the output.

If JSR 3 is called and the conditions on rung 1 are not true that would turn O:6/2 OFF, or just leave it on if true.

Since these are after it is controlled on ladder 2 maybe the programmer wanted to make sure it gets turned off on 4 tries before the end of the program scan.