TapHome Bus (RS485)

TapHome modules communicate via proprietary protocol, over RS485 physical layer.

The basic principle is to provide collision-free reliable and prompt communication between central control unit and modules.

RS485 physical layer

Bus connections must be powered by the same potential, because there is neither galvanic, nor optical isolation. While RS485 is based on voltage difference between A and B wires, it provides high protection from noise even on long twisted pair of wires.

Core provides 3 bus connections. Maximum number of modules on each bus is 30. Altogether 90 modules on one control unit.

Topology and wiring

Parameter	Recommended	Note
Cable length	< 600m	Up to 1200m in special conditions.
Cable type	AWG 20 - 22, Twisted pair	Shielding is usually not neccessary but nice to have, in case there is unexpected problem with noise.
Change to Topology view in ETS project:	Daisy chain, Tree	Daisy chain is optimal for long distances, tree can be used up to 600m, and star is also acceptable. Ring topology is prohibited.
Termination resistor (between A and B wires)	100 Ohm at the most distant point of the bus	In most cases, communication works well even without termination resistor, but is much more prone to noise.
Max number of modules (connected on 1 bus)	< 32	In good conditions (minimum noise, short wire distance, daisy chain topology), there can be up to 100 modules connected on 1 bus.

Parameter

Recommended

Note

Cable length

< 600m

Up to 1200m in special conditions.

Cable type

AWG 20 - 22,

Twisted pair

Shielding is usually not neccessary but nice to have, in case there is unexpected problem with noise.

Change to Topology view in ETS project:

Daisy chain, Tree

Daisy chain is optimal for long distances, tree can be used up to 600m, and star is also acceptable.

Ring topology is prohibited.

Termination resistor (between A and B wires)

100 Ohm

at the most distant point of the bus

In most cases, communication works well even without termination resistor, but is much more prone to noise.

Max number of modules (connected on 1 bus)

< 32

In good conditions (minimum noise, short wire distance, daisy chain topology), there can be up to 100 modules connected on 1 bus.

TapHome Bus protocol

TapHome Bus protocol is based on round robin principle, where Core is master, and all connected modules are acting as slaves. In standard operation, module can only send response to a query. This provides a few benefits:

*Collision-free communication

*Similar performance regardless of number of changes that are being transmitted

*Immediate error detection

TapHome Bus (RS485)

RS485 physical layer

Topology and wiring

TapHome Bus protocol

Communication parameters

Round robin

Packet structure