which hardware to use?

[Invention1]

So here is the hardware problem:  I need to have a black box that can accept an RTD signal at cryogenic temperatures, (just one RTD at -200C)  and create two relay outputs based on that signal.  20 years ago this meant a board inside a computer, or something running RS485.  That isn't where I want to go today - I'd rather have an external  black box that either plugs into USB or ethernet, and can accept some direction from a stock bare bones PC running whatever the flavor du jour of windows happens to be this week. The PC will be the user interface.

Now, the black box could be as fancy as a dedicated controller like the Single Board RIO, that accepts a few inputs and does all the rest of the control, however, i doubt we'll have the budget for that.  I am thinking that Single Board RIOs cost a minimum of $700-1000 a pop, and we want to have a budget of maybe $150 - $500 a pop.  It would be ideal if the black box could at least be a dedicated PID controller, that would accept setpoints over a computer interface, but it could also be as crude as a relay board and an analog input board (That's what we have been using for a long time).  The crude relay/AI board must use the PC for all the smarts, which puts us at the mercy of Microsoft and thier tendendcy to hang, however it has worked OK for quite a while. 

A PID controller would have one advantage, in that if it didn't get any signals from the computer after a hang, it would continue at the last recieved setpoint until someone restarted the spasmic brainbox.  

The PC Computer's role would be to update the setpoints on the black box periodically, as the parameters change over time, and then log the data coming out of it, eventually dumping it into Excel. 

I've looked at several different packages offered by NI, but haven't dialied in on one that would accomplish the task at the price point I am looking for.  Mostly I keep finding things that have a lot more capabilities, like 64 DO points, or whole web servers, or other expensive options that jack up the cost. 

Can anyone direct me toward a hardware unit that might fill the bill? 

[charris]

Invention1:

Just to start off and clarify, I can pretty much guarantee none of our hardware platforms are rated to operate at temperatures that low. I'm assuming the measurement/control device will exist ouside of the -200C environment?

If the black box is going to be a human control interface that logs data, changes setpoints, and doesn't need to perform time-critical funcions (at least not with sub-millisecond accuracy/precision), then your best bet (especially on a budget) is going to be to use a PC/DAQ board for your control application.

You could use control functions in LabVIEW to monitor process variables and change setpoints, and a multifunction DAQ card (like an M or X series) would allow you to read your RTD (depending on your wiring/excitation setup) and control DIO lines in software time to operate relays.

If you're looking for an external controller for PID and data collection, then the only real option is going to be CompactRIO or NI Single-Board RIO, both of which are likely to fall outside of your budget purvue. If you can live with software timing on your control, then your best bet is probably an M or X series DAQ board.

What is the wiring configuration on your RTD (2-wire, 3-wire, etc), and how much power do your relays require ro operate?