HI I know it's a bit late in this discussion but I thought I would throw my tuppence in ref PC PLC's. We use a version here in all our control systems for exactly the opposite reasons others have mentioned. We design and build underwater robots and the product used is from a company called Softplc www.softplc.com which we have over 12 years experience with. It does allow live editing, the only thing you have to stop the PLC for is to save the edited code. It is fully compatible and has all the features of Allen Bradley PLC5 including a range of protocols for comms including DF1. It runs on any Dos, is compact (obviously) allows data logging, storage of setups in data files, Full web server capability with an embedded TCP/IP stack, retentive data (Kind of, you need to deliberately save the datafile) and reusable and extendible code. ie If there are any major routines used regularly, (like for us, a decent analogue filter for sensors), we write it in C code and build it into Softplc as a new instruction. It's as rugged as hell which is just as well for us and we don’t have a problem with obsolescence BECAUSE it’s based on PC architecture. We still support systems built over a decade ago by using newer and faster boards. (All our stuff is PC104). We occasionally get bitten on the bum by some old code that expects a slow machine (That’ll be Moore and his sodding laws). As time progresses, the electronics get smaller and convergence pushes more functionality onto the same board which means we occasionally have to write a new driver but again it’s no big deal. The biggest reason for us to use Softplc is that the hardware is tiny compared to the equivalent PLC. (all our electronics has to go inside pressure housings.) Another biggie is that I can write an application in a couple of days for a new robot and then use a SCADA package to create some cracking control screens. I can’t do that with C code. The code can also be understood by others. (ever tried to read someone elses C code….ugh) The only problem with SoftPlc’s in general is that the I/O cards from most manufacturers don’t incorporate proper isolation so we had to create a set of PC104 boards to do that for us. With underwater kit, keeping water out of connectors is impossible so the isolation cards prevent the expensive brains of the system from blowing up. We considered moving to PCMCIA cards for the product but although the electronics is there, the connectors on the cards are shoot so we are sticking with PC104 for the next few years. The reason I came to this site was to find information on an obsolete PLC used in a Russian Crane. You may not like PC architecture, not many do but at least it will still be around in 20 years time even if it is a bit smaller.
