RS485 Signal Tester - Schematic?


Thread Starter

Warren Postma

does anyone have a schematic for an RS-485 signal tester. It would be nice to have data signal flash an LED (perhaps with a pulse stretcher so that brief activity is more visible), an "out of range voltage" indicator, an "open" and "short" LED, etc.

Has anyone built such a beastie? I know B&B Electronics sells one, but theirs seems minimal, and for the price ($180 US) I thought I might try to make one myself (more because I want to learn about RS485 than to save the $180).


Johan Bengtsson

In RS485 you have two signal wires, they have some certain voltage when there is no activity, this can easily be calculated looking at
the voltage dividing in the terminal resistors (I leve it as an excercise to you to caclulate it).

When there is activity the voltage polarity between those wires ALTERNATE depending on the bit sent, ie the low voltage wire is forced higher than the high voltage level. I don't know the exact voltages from memory but could be looking it up on a real network (profibus DP).
It should be enough to detect activity by feeding those directly into a comparator.

There should *always* (except during the short "crossing") be at least some difference in voltage between those wires.

One note: depending on protocol there might be activity all the time even if nothing interesting is transferred, like the token passing in profibus.

/Johan Bengtsson

P&L, the Academy of Automation
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]

Paul N. Leonard

We have used two LEDs and a single resistor to indicate activity on the line. Use one red and one green LED. The LEDs are connected in parallel but in opposite directions. One LED junction goes to the + signal line. the other junction goes to a resistor. Since it is a 5V signal, a 270 Ohm resistor (1/4w) will work just fine. The free end of the resistor connects to the - signal line. The tester won't severely load your line and provides polarity information as well as activity indication. It's cheap and after a while you can "see" normal traffic, collisions, and reverse wiring.