Does anyone know of a 4-20 ma loop signal transmitter that will allow manual push button control of the output. One button will increase, second button will decrease. Preferably should work in .01 ma steps. I'm hoping someone out there knows of something COTS in lieu of my designing and building an up/down binary counter and feeding the outputs through an R / 2R network to generate a variable voltage signal. This is to be mounted in a remote operator panel with start and stop buttons. Problem is the panel must be Class 1 Div 1 explosion proof, so a panel mounted pot isn't exactly kosher. This is going into an A-B 1200 system, but the solution to this issue is probably generic to almost any modern PLC.

Thanks in advance.

 

There are some cheap PLCs that would do this. Just find one with an analog out and write a small program. That's cots and not very special. Some would even have the buttons.

Regards,
cww

 

How about a little 3" Modbus/RTU Master LCD/keypad connected to a Modbus/RTU slave 4-20mA output module?

4-20mA Output Module...
ADAM-4021 or ADAM-4024:
http://www.advantech.com/products/ADAM-4021/mod_1-2MLDDH.aspx

3" Modbus Master LCD/Keypad...
http://www.automationdirect.com/adc...ls_-a-_Keypad_Bezels_(3_inch_only)/EA1-S3ML-N

 

By explosion proof, you mean your panel is a cast and bolted type such as Killark or Crouse-Hinds, right? Check the Killark catalog, page C46, G Series Controls Specialty Operators. Add a suitable pot and pot/4-20mA transmitter and you have your analog input to the PLC. If you want to feel smart and cheap, get a 3-terminal voltage regulator IC and use the pot as a voltage divider to a voltage analog input.

If you want to stay with increase/decrease push buttons, don't bother looking for a PB/current transmitter. Use the push buttons as discrete inputs to the ML1200 and write the up/down counter in ladder logic. OK, that's what Curt wrote but you already have the PLC.

Mike