Bus Extender — Wiring and Bus Termination

The Bus Extender connects to the Core via USB and adds 4 RS-485 bus ports to the system — for the TapHome bus as well as Modbus RTU. Unlike the Core, it has no termination resistor built into any of its ports. Where the resistor goes depends on where the Extender sits on the line.

Ports#

• Bus 1, 2 — non-isolated (A, B)
• Bus 3, 4 — galvanically isolated (A, B, iGND)

Use an isolated port if at least one of these applies:

• A device on the bus is powered from a different source without galvanic isolation (for example a heat recovery unit).
• The bus cable leaves the building where the Core is installed (for example to a garden house). Power the devices on the isolated bus from a separate power supply in that case.
• The cable is long and the modules on it have higher consumption — because of the voltage drop on the cable.

For low-consumption modules and cables up to 150 m, a non-isolated port is sufficient.

On an isolated port, it is recommended to also connect iGND to the ground (GND) of the devices at the other end of the cable, so that A and B are at the correct voltage potential. Communication should work even without iGND connected.

Up to 3 more Extenders can be chained through the Extender’s USB port. The Bus Extender requires OS Image 2.11 or higher.

Bus Termination#

The 100 Ω termination resistor goes between A and B at both ends of the line. The Core has it built in on every bus, the Bus Extender on none of its ports. Where to put it depends on the topology — the same for isolated and non-isolated ports:

• Extender at the end of the line: 100 Ω on the Extender’s A and B terminals and 100 Ω at the far end of the line.
• Extender in the middle of the line (two branches leave its port): 100 Ω only at the ends of both branches, not on the Extender’s terminals.

Shielding#

On isolated ports (Bus 3, 4), the cable shield is connected to GND at the remote power supply (or to the GND of the module located near the supply); it is not connected at the ends of the cable. General shielding rules and cable selection are in serial bus topology.

Wiring Diagrams#

Extender in the Middle of a Tree#

Two branches leave the non-isolated Bus 1 port. The Extender sits between them, so no resistor goes on its terminals — 100 Ω is at the end of each branch. The modules are powered from the same power supply as the Core.

Long Cable from an Isolated Port#

The remote modules are powered from the power supply at the Core through a long cable. The isolated Bus 3 port carries A, B and iGND; at the remote end, iGND is connected to the GND of the modules. The Extender is at the end of the line, so 100 Ω goes on its terminals and at the far end.

Remote Section from a Second Power Supply#

The remote modules have their own power supply. Only A, B and iGND leave the isolated port, with iGND connected to the GND of the second power supply; the +24 V of the two supplies is not connected. 100 Ω goes on the Extender’s terminals and at the end of the line.

Modbus RTU Device with Its Own Termination#

The device with its own power supply is on an isolated port, with iGND connected to its GND. 100 Ω goes on the Extender’s terminals; on the device side, enable its built-in termination. If the device has none, put a 100 Ω resistor between its A and B terminals.

Galvanic isolation of the bus with a separate isolator is described in optical bus isolation.