# RS485 Signal Tester - Schematic?

W

#### 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).

Warren

J

#### 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

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