cable for Modbus RTU

i have an 8pin mini din to RS485 converter 1763-NC01, that i will use to communicate on MODBUS RTU i was just wanting to confirm what sort of cable would i need to use, i was thinking of 2 core shielded or does it have to be a specialist cable?

rob

It's ordinary RS-485, so any cable that meets that spec will work.

I don't recommend using a two-conductor cable because that forces you to use ground or the shield as the common reference. That's cheap and works much of the time, but if you have difference in earth potential between the two nodes, or if there's a lot of noise in the environment, you're going to have current flowing in the shield. That is never good for serial comms.

A-B has recommended Belden 3106A for many years for DH-485. It's a "1.5 pair" with a twisted pair (orange/ white-orange) and a single blue conductor for an isolated common.

For short runs, I know that ordinary CAT5 cable is used very often for RS485. Use one pair for signal, and short another pair and use them both for COM.

i was working under the assumption that it was just a 2 wire comms, as the allen bradley RJ45 plug has just 2 screw terminals.

the reason i asked about using shielded cable is because i am just setting up a training rig with a ML1500 and a PF4 so don't want to buy a huge amount of cable when all i would need is about 1 meter.

we do have a few drums of Cat5 if this would be better than shielded cable?

regards

Rob

RS-485 can be 2 wire or 4 wire - you need to check which.
RS-485 is designed for 100 ohm impedance wire so CAT5, CAT5E, CAT6 is a great choice. If running near anything industrial (motors, 480V, etc), running shielded cable is a great idea. Most Cat5E is 24 ga so length is limited compared to the larger (and more expensive) RS-485 specific wire. When shielding, be sure to only 'earth' the one end of the wire to avoid any ground loops. If all you're doing is testing, unshielded CAT5 will be fine. And don't forget your RS-485 terminating resistors...

is this a 120ohm resistor?

is there a AB pdf file for this type of installation?

rob

You can read until your blue in the face about RS485.

See links below for starters

http://www.bb-elec.com/technical_library.asp#RS422

http://www.google.com/search?q=rs48...s=org.mozilla:en-US:official&client=firefox-a