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Old May 31st, 2002, 09:57 AM   #1
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DickDV is offline
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Beginner-help sizing PLC w/large I/O

While I am very familiar with relay based ladder logic, I have limited experience with PLC's, having concentrated entirely on variable speed motor/drive applications for 15 years.

I would like to consider using a PLC for personal hobby/home use in order to become more familiar with programming. I am looking at about 100 DC signal level inputs and about 100 dry contact or open collector outputs. Response time can be slow--.5 to one second worst case. There would be no analog I/O and no math functions--only lots of logic and a small amount of timing.

Since cost is a factor, my instincts are to buy a minimal processor and load lots of I/O on it. Something tells me that this might not be smart since, if I understand correctly, minimal processors may not have enough internal relays, timers, etc. to do the job with that much I/O.

Also, I was thinking that the Toyo built PLC (marketed by Automation Direct, formerly TI, GE series 90, and possibly Siemens or others) would be a good choice. (Cost again!)

My questions are: how do I approach sizing a PLC and selecting a suitable unit given the above? What traps should I avoid? What about availability of surplus/used equipment?

If you prefer, a response can be made directly to my email:

Thanks for your help. I really appreciate it.
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Old May 31st, 2002, 11:25 AM   #2
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Simple, if your PLC of choice can handle this amount of I/Os, it will certainly handle any logic you can imagine.

simple logic that is. If you try to do some DAQ and complicated math functions with it... the DAQ will be minimal...
If it looks like a Cat...
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Old May 31st, 2002, 12:30 PM   #3
Tom Jenkins
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In my opinion, you can't beat the Koyo DL-250 PLC from in terms of bang for the buck. It can handle the I/O you need, the programming software is easy to use and reasonably priced, and their tech support is responsive and easy to reach.

If you want to go bigger the 305 and 405 series are also reasonably priced.
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Old May 31st, 2002, 12:55 PM   #4
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Thanks! and what about used/surplus

Do the Koyo PLC's appear in surplus/used sources and is this practical? (A new system looks like about $1000) And is this PLC relabeled with other brand names as I suspect? I'm particularly hesitant to deal with surplus sources for the main processor. Maybe just the I/O?
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Old May 31st, 2002, 01:17 PM   #5
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My first thought since you need want many I/O was to find used system like a GE Fanuc 90-30, these are available on ebay alot. Any base 10 should come close to what you want without paying a big fortune, I have seen fully loaded base 10 331 cpu with 5 input and 4 output cards and power supply go for $350 or in that range. There are a few base 5 CPU 311 there now:

You do know that to program any of the SOFTWARE and cable/adapters will be needed. Thats another reason I suggested finding a used Fanuc because Versa Pro isnt that costly. I dont remember exact price but I paid around $300 in the last few months.

The manuals you can get at

Best of all if you get tired of it any time you can always put on ebay and recoup some if not all your money.

Note: you are suppose to upgrade up to firmware 10 to use Versa Pro, the firmware chip is less than $100 but I found out the software will still work even with the older firmware chips. If not going to use in anything but a trainer than may not have to purchase.

I dont know of another brand that you could find that would give you the I/O, software etc for the price. We talking all the major features, win based software, numerous I/O, analog available, servo motor control available...the list goes on and well under a $1000.

If you need alot cheaper than that then look Koyo stuff and go way lower on the I/O but expand later.

Also Siemens and some others have Starter kits that include cpu, software, cables and manuals. Usually under $500 depending on the cpu.
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