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We're looking to install some temperature/humidity monitoring in an existing facility. Installing new cabling would be very expensive because of the nature of the plant. Using the existing cat-5e structured cabling would cost nothing. The product of interest is the Teracom TCW210-TH plus TSH300 Modbus modules, if anyone is interested.

I have a few questions concerning the viability of running Modbus (RS485) over a cat-5e UTP cable plant. I'm unfamiliar with these technologies. With Google I've found somewhat contradictory statements like "not guaranteed above 30m but known to work over much greater distances", and elsewhere references to ~200m being where Modbus over cat5e becomes flaky. Anyone have experience on this front?

Firstly, is it likely to work reliably with a total bus length of around 120 meters? I'm aware that Cat-5e characteristic impedance is 100 ohms, whereas RS485 is spec'ed for 120ohm STP cable.

Secondly, does RS485 reject interference between twisted pairs in a single cable, in much the same way that BaseT Ethernet does? In which case, could I construct the ModBus daisy-chain using "Y" adapters plugged in to a single wall socket, with the signal coming in on one pair and going out on another? (And similarly in the wiring cabinet). You can certainly run two 10BaseT or even 100BaseT Ethernet links down a single cat5e cable in this way.

The alternative would be to use an Ethernet connection into each room in the facility and ModBus purely for linking modules within that room. The drawback is that the data would then be spread out over several Ethernet devices, rather than all conveniently aggregated in one place. The TCW-210 appears to do almost everything we want in its own firmware and web interface, so saving us on programming costs and time.

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marked as duplicate by RoyC, PeterJ, Voltage Spike, Dave Tweed Jan 28 '18 at 12:48

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  • \$\begingroup\$ 120m is short distance, also you won't face bankruptcy if you buy the correct twisted pair cable: Liycy TP 2x2x0.5 . It would cost arround 150$ \$\endgroup\$ – Marko Buršič Jan 24 '18 at 13:36
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    \$\begingroup\$ Why not Modbus TCP/IP? \$\endgroup\$ – Tyler Jan 24 '18 at 13:43

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