We have some existing runs of Cat5E STP shielded ethernet cable that we would like to run Modbus on. The cables have RJ45 shielded plugs on them. The length between devices (SquareD variable frequency drives) is less than 50' and the overall length is approximately 120'. There will be 16 devices on each segment. Each device has a "T" that terminates with 2 female "RJ45" jacks and a short cable with an RJ45 plugt that plugs into the RJ45 jack on the VSD. My question is will it work on this cable?

 

You're asking because ? ?

your comm link is RS-485 and CAT5E there isn't a large experience base using ethernet cable for serial links?

Or you're doing Modbus/TCP and you're not using switches and conventional ethernet topology and components and you think you can avoid the cost of cable and switches for a standard topology?

If you're doing Modbus over RS-485 that short length of cable on the Tee's plug is what could cause you problems. If you were running Profibus over 485 at >1Mb/sec, you'd likely have problems, but if you're doing RS-485 at 19.2k or thereabouts, I suspect it'll probably work OK.

If you're doing Modbus/TCP, I suspect you'll have all sorts of problems, but with the cost of ethernet components being what they are, I've never experimented, I just use what's needed. So I honestly don't know what you'd run into.