The quiet bus and its two ends
A terminator of 120 ohm goes at each of the two ends of the trunk and nowhere else, to stop pulses bouncing back. And when no driver is on, the pair floats, and only bias gives an idle bus a level that a receiver can read.
Why the ends
A pulse on the trunk travels both ways. Where the cable ends open, part of it reflects and comes back as ripple on the next edge. A reflection comes from a change of impedance, and the cable's is about 100 to 120 ohm.
A 120 ohm resistor across the pair at an end matches it, so the pulse finds no change. Both ends need one, because either end may be where a pulse arrives.
Two, and no more
The Modbus guide is blunt: never more than two terminators on one pair, and none on a derivation cable. Caps are in parallel, so each extra one lowers the load the driver sees. Twelve of 120 ohm are 10 ohm, and the driver is specified for 54.
How much the ripple matters depends on the driver's edge time against the cable's delay, and a fast chip fails TI's test at a metre or two. A slow link survives because a UART reads each bit in the middle, after the ringing has died. The reflections are still there.
The pair with nobody on it
When every driver is released, nothing holds the pair, and noise decides what it reads. Terminators hold it near 0 V, inside the receiver's ±200 mV dead band, where the datasheet promises no output.
The MAX485 and MAX3485 failsafe covers open inputs only. A terminated pair is not open, so it does not apply.
Bias
Bias is a deliberate small tilt: one resistor up, one down, so the idle pair points at a mark, a 1. Modbus asks for one pair, 450 to 650 ohm each, at one place on the bus, and none on the other devices. Which wire gets the pull-up depends on the lettering: the Modbus guide pulls up the wire it calls B, which our modules call A. Bias reduces the number of devices allowed by 4. With bias present, the guide suggests 120 ohm in series with 1 nF as the terminator.
Common mistakes
- A terminator on every board. Eight of them are 15 ohm. Fit one at each end only.
- Counting on the failsafe. It sees an open input, and a terminated pair is not open. Give the bus bias.
- Bias on every device. The pairs add in parallel. Put one in one place, normally at the client.
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