RS-485 bus and cabling

MSCB uses half-duplex RS-485. The standard cable is a 10-conductor ribbon carrying the differential pair, common rails, reset/interrupt lines and two reserved conductors.

10-wire assignment

Conductor	Signal
1	Data A
2	Data B
3	−15 V
4	+15 V
5	+5 V
6	GND
7	Reset
8	Interrupt
9	Reserved
10	Reserved

Only A, B and a reference/ground are required when a node is powered locally. For long or noisy runs, use a shielded twisted pair and isolation where needed to prevent damaging ground-loop current.

Loading, length, and termination

Using 1/8-unit-load transceivers allows up to 256 devices on a segment. Cable length ranges from roughly 100 m to kilometers depending on baud rate, cable quality and topology. The documented maximum bus speed is 345,600 bit/s.

Terminate the two physical ends of a long bus. Use about 100 Ω across A/B and bias resistors at the powered end so an undriven bus has a defined state. Do not terminate every node, and avoid long unterminated stubs.

Serial character

The serial waveform consists of:

1. a negative start bit;
2. eight data bits, least significant bit first;
3. a ninth bit, set only for address-cycle characters; and
4. a positive stop bit.

Line B is the inverse of line A. Receiver logic depends on the A/B differential polarity of the installed transceiver.

Practical commissioning

• Verify A/B polarity before scanning.
• Confirm one common reference or intentional galvanic isolation.
• Install end termination only at the trunk ends.
• Start at a conservative baud rate on unknown cabling.
• Check that node power remains in tolerance at the far end under load.
• Assign unique node addresses before connecting independently configured groups of nodes.