Need help on choosing cable to connect rs485 devices to PLC

again back to rs485... I have run some loops in very bad areas just using cat5 cable or whatever cable was lying around at the time. Not ideal but I dont get intermittend drop in Serial comms. The only times I do get issues is when the max and min distances between slaves are not adhered to. Personally think those comms would work well on a clothes hanger

chud said:

Personally think those comms would work well on a clothes hanger

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Some anecdotes. I show up and that RS-232 point-to-point, is wired with about 100 feet (30m) of Romex (flexible plastic sheathed cable for house wiring in the US). It worked (and RS-232 is not a diffential technology. Spec says 232 at 15m or 50 feet.



1200 feet of CAT 5 cable run for a remote pump house for email alarms, Modbus/TCP, internet to download a file if needed. back in the 10Gb era. Worked flawlessly and the Ethernet spec is 100m max (which is definitely a consideration with 100Mb and 1Gb speeds; max limit has to do with the propagation speed of the signal.


But I get really fussy about multidrop RS-485 (Modbus RTU) and insist on shielded twisted pair but comm is usually down in the 38K baud range. I also put an isolator at the master and another isolator every 3 or so devices from an experience where a near lightning strike took out the whole string.