UTP Category-5 LAN Cable Inquiry?

[ritchie]

Hi,

I am working on a project that will utilize the RS485 multi-drop network. With my search on the internet, most of the responses and materials suggested to use a cable that AWG #24 with a characteristic impedance of about 120 ohms.

My concern really, what would be the typical characteristic impedance of a Cat-5 LAN cable? Can I use this cable for my RS485 netowrk? what would be the typical wire speed for Cat-5 LAN cable?

I need your comments on this?

Thank u.

[MADIS - Alexandre Maurer]

You can calculate it yourself with the characteristics given by the cable manufacturer and your operating frequency




Where R' is the total resistance of your line (Ohm)
Where L' is the inductance H/Km
Where C' is the capacitance C/Km
Where G' is the perditance S/Km
Where w is the pulsation rad/s (2 * PI * f)

Terminate your line with a resistor equal to the Zc value. (only real parameter, not imaginary cause it will be to hard to do LOL)


if you work at about 1MHz and more, the characteristic impedance of your cable will be SQUARE_ROOT(L'/C') wich will give you ~100Ohms for a CAT5 cable.

But with RS485 i think you will work at 250kbps... so I recommend to you to use 120Ohms or 150Ohms at the source and at the receiver with a CAT5 if you want.

Also check with your oscilloscope if you have standing waves. The power must be full dissipated at the receiver end. Adapt the termination resistance.

[dyeatman]

I have used CAT-5 extensively for RS485 with no problems. I have used "star" configurations, serial multidrop and combined. It has worked fine in every case. The only word of caution I might add is to steer clear of noise sources like flourescent lights. I had a case where I got too close with one run and the devices on that leg were intermittant. It took a while to find.

Good Luck
dave

[ritchie]

[ritchie]

[ritchie]