Micrologix 1100 onboard analog question

[Michael Lloyd 85]

Typically I use some Micrologix and a lot of SLC, Contrologix, Compact Logix. We decided to use a Micrologix 1100 on a small project that doesn't warrant the horsepower of the other PLC's. It has a few cards added to it including one 4 point analog input card that has a single point left on it. We need to add 2 more analog points and rather than add an AI card I was thinking that I could use the 2 on board analog points that the ML1100 comes with. But... our stuff is all 4-20mA not 0-10VDC. We looked into buying a couple of 4-20mA to 0-10V converters but they cost as much as it would cost to buy another AI card. The downside to adding the AI card is that real estate is getting tight in the cabinet. I've never really bothered with the on board AI's so any information about them would be appreciated. Am I understanding the on board AI's correctly? 0-10VDC?

[pcmccartney1 141]

Yes, it is only 0-10VDC. You could wire in the 4-20mA signal and install a high precision 250 Ohm resistor to read voltage then scale appropriately in the ladder.

[PLCMentor.com 60]

You have to be careful using the on board analogs. The resolution is crappy (10 bits) which may be ok for certain things. You could probably get away with the 250 ohm resistor, but that would only use the 1-5 v range of your 0-10 analog input cutting your resolution in half... and did I mention that the resolution is crappy? In my mind, if you need 2 more analog inputs then its worth adding the extra card. You always want to leave some spare capacity anyway. Your cabinet space problem is difficult. I dont really have any good ideas on that. Also keep in mind that the 1100 can only take 4 expansion modules. So you may be backed into a corner there depending on the number of modules you have in your system.

[Michael Lloyd 85]

Unfortunately I've already used up all of my "card slots" and I can't add another analog card. Adding the two analogs wasn't my idea and I don't think we need them (4 tanks, common headered together, two level transmitters now and he wants to add two more. Gross overkill for what we are doing). The poor resolution won't be my problem :o) I went with 4-20mA to 0-10VDC converters rather than jack with resistors Thanks!

[TimWilborne 108]

Good choice

[rguimond 1]

If you're REALLY stuck, and if you can get by with a slower reaction time, try sharing the analog input. Here's what worked for me... Install a 8-pin octal relay with NO & NC contacts. Connect the common terminal to the analog input. Connect the analog signal from one device to the NC contact and connect the NO contact to the signal from the second device. Using an unused discrete output, energize/de-energize the relay coil at a suitable rate (I used 2 seconds). After the coil changes state, wait 500ms, then read the analog value. Do the same thing with the other device when the relay changes state again. The relay will actually have two sets of contacts, so you can use this method for connecting 4 analog devices to two analog inputs. Of coures, this may introduce some error and may require rescaling (my application didn't), but it's a quick'n'dirty solution to an otherwise insurmountable problem. Give it a try and let us know how it worked.

[billysmithde2 1]

Spectrum makes an 8 channel analog card 1762sc-IF8u. I have had to replace some of my 4port cards with this to get more channels when the 'rack' is full.