One trunk, many taps

One line, short taps

An RS-485 bus is one trunk cable from the first station to the last. Each station joins it directly or through a short stub. The Modbus serial guide says the same: one trunk, with devices on it directly or on short derivation cables. It is not a star, with a cable from every station to a hub.

A stub is a short branch off the trunk, and a long one reflects. TI's note states the intent: the driver should see only one transmission line.

How long a stub may be

RS-485 gives no single number. TI says "as short as possible", and a fast chip wants shorter stubs than a slow one at the same baud rate. Modbus does give one: derivations never more than 20 m, and 40 m divided by n for a tap with n of them. The mark on the bench's slider is the bench's own.

Unit loads, not boxes

A receiver draws a little current from the line. One unit load draws at most 1 mA at +12 V, about 12 kΩ, and the baseline allows 32 unit loads on a pair.

So the count is of loads. The MAX487 has quarter-load inputs and allows 128 transceivers; TI's eighth-load parts allow 256. A MAX485 or MAX3485 board is one full load. Terminators are not counted, but Modbus bias takes four off the number allowed (lesson 6).

An address is not a position

A Modbus address, 1 to 247, is set in the station with switches or a menu. Stations 5 and 2 can stand side by side. Two stations with the same address both try to answer, and their replies collide.

Common mistakes

• Wiring a star. A long cable from each station to one hub makes every one of them a stub. Run the trunk past each station and tap it there.
• Counting boxes. The limit is 32 unit loads, so a mix of parts has to be added up, not counted.
• Setting the address from the position. Numbers given in the order the stations stand break when one is swapped. Set it in the station and write it on the label.