We've used A-B, Panasonic, Automation Direct, Omron, and Siemens PLC's.

The PLC approach does have its advantages. They are industrial quality, with various input (24VDC, 120VAC, analog in, etc) and output (24V, relay, analog out, etc) options, while the C-programmable stuff is typically bare boards with TTL (5V) inputs and outputs. If you need 4-20mA, you'll have to check that the PLC has an option for it.

Some do have built-in PID; we've done PID temperature control. If you want to do PID, make sure it's built in.

Older PLC's are programmed solely in ladder logic, which is a fine fit for combinatorial logic ("when these inputs are in this state, do this"), but sucks for everything else (e.g. sequence of actions). Newer PLC's support the IEC61131 standard, which supports five langagues: ladder, function block (ladder written with AND/OR instead of wires), structured text (our favorite; a mix between Basic and Pascal), flow chart, and ladder assembly.

PLC's are horrible for any kind of data handling, so if you do use them, that recommended approach (use PLC as PID controller, don't do NMEA conversion) makes sense.

The price will be higher, and you'll have to pay for programming tools (A-B's are very pricey, and have copy protection; Siemens, Aromat, Automation Direct are more reasonable) or know someone who has them.

Some companies have starter deals which include a PLC and software. Aromat has run specials on PLC's and Operator Interfaces occasionally. Siemens runs them often (including both PLC and Operator Interface); you will need to contact your local distributor.

I recommend looking at [link|http://www.sea.siemens.com/automat/product/plc/auov.html|Siemens], [link|http://www.nais-e.com/plc/|Panasonic], and [link|http://www.automationdirect.com|Automation Direct] PLC's. Automation Direct has all their pricing on line, and it's competitive. (A-B does has on-line pricing, too, if you register, but it's quite high, even after a 30% discount). Then you can use resources such ebay and [link|http://www.plccenter.com|PLC Center] to see if you can get surplus or used equipment at much better prices.

--Tony