Just two more analog inputs

[cch1955]

What is the easiest way to add two more analog inputs to the t100md2424? I was hoping for something cheap and easy.

[support]

Use a T100MD888+ PLC to add 8 more analog inputs to your T100MD2424+ PLC. You can use RS485 communication to read from the slave T100MD888+.

[Joel Moore]

Sounds like a job for an analog multiplexer.  I'm not familiar with the design of such circuits but they're pretty common I think.  For example, they're used in cheap multi-input video frame grabbers.  4 inputs -> 1 A/D via an analog multiplexer.

Of course you would need to figure out a way to synch it with the PLC.  Meaning you would need to control which input of the multiplexer is active using an output from the PLC so that you know which input you are sampling.  The timing of that could be tricky, especially if you can't tolerate a slow sample rate.

Another option (other than what support mentioned above) is to get a serial analog input module.  ICP has quite a selection of these but I don't know what your definition of cheap is.

[cdenk]

Check for my posts several months ago. I'm not a electronic guru, and it was a struggle, but has been reliable for a month or so. I used the TTL output of the EXP... board to address the multiplex chip. If need more info let me know.

[cch1955]

I currently am changing the software so I don't need it. I had planned on adding a small relay to select
the analog device I was wanting to see values on. I only needed a snap shot of the value, so real time was not an issue. thanks to all.

[cdenk]

One of my concerns with mechanical relays was repetitive cycles, at maybe 10 cycles per second. I didn't need that sort of frequency but was convient. I have changed out little relays on PC boards in recent time when a contact went bad.