Can you use a cross over null modem to communicate from a plc5/15 to a laptop pc using db9m to db9f plugs
using rslinx and rs logix5.

nope ... sorry but you can't get there from here ...

the DB-9 on the front of the PLC-5/15 is ONLY used for DH+ (Data Highway Plus) communications ... you need something special (special as in expensive) to make the connection ... for a laptop computer, this is usually a PCMK card - and the special cable that goes with it ...

suggestion: post again and tell us EVERYTHING about how your PLC system is connected together ... sometimes it's possible to do a "passthrough" connection using a spare SLC-5/04 processor on the same network ... do you have anything like that available? ...

USUALLY when we get someone asking your question, it's because they've recently purchased a PLC-5/15 processor on ebay to play/learn with ... unfortunately the PLC-5/15 (and its close cousin the PLC-5/25) are called "classic" processors ... the biggest problem with them (for a beginner) is that they don't offer the RS-232 communication port the way the newer "enhanced" PLC-5 processors do ... is this the boat that you're in? ...

and welcome to the forum ... wish we could have given you some better news on your first trip ... but at least now you know ...

The only "cheap" way into that PLC is to use an older desktop PC with an ISA slot and a 1784-KTX or 1784-KT off of ebay. You'd better not have anything newer than a socket-7 motherboard if you want to use the KT.

Cheers for that I have used the null modem approach on SLC processors OK, will have to get the company to buy the right kit from Rockwell for PLC5/15.The PC is in a hostile environment so any devices taken in have to stay there.On the bright side you have saved me some time and effort making up the lead.Thanks for the welcome.

On the bright side though, DH+ (the comm. protocol this PLC uses) lets you put your programming terminal FAR away from the PLC if you want. So you could monitor/program the PLC from outside of the hostile environment if that's something that would be of benefit to you

one other approach to consider ... swap out the "classic" PLC-5/15 processor for an "enhanced" PLC-5/20 processor ... that will give you a built-in RS-232 port ... in some situations (especially where spare processors are already available) this might be a cheaper option than buying new hardware to communicate with the older system ...

word of warning: the PLC-5/20 is "enhanced" and DOES have a built-in RS-232 port ... but ... the PLC-5/25 (note the higher number) is an older "classic" and does NOT have a built-in RS-232 port ... so why did AB "skip around" with the model numbers? ... your guess is as good as mine ... just be sure to check the features before you buy anything ...

Swapping the processor is not an easy option as the paperwork involved in that type of modification on a working plc is time consuming(weeks if not months), I can get the comms kit bought in under 1 week, and this is the way to go(large multinational).I was just looking for a quicker fix to earn some brownie points.Thanks for the support though it is appreciated.