User Manual

TMCM-3351 Hardware Manual • Hardware Version V1.00 | Document Revision V0.91 • 2017-JUL-11

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7 Communication

7.1 RS485

For remote control and communication with a host system the TMCM-3351provides a two wire RS485

bus interface. For proper operation the following items should be taken into account when setting up an

RS485 network:

1. BUS STRUCTURE:

The network topology should follow a bus structure as closely as possible. That is, the connection

between each node and the bus itself should be as short as possible. Basically, it should be short

compared to the length of the bus.

c:>

node

- 1

node

Host

Slave Slave Slave

RS485

termination

resistor

(120 Ohm)

termination

resistor

(120 Ohm)

}

keep distance as

short as possible

Figure 7: RS485 bus structure with termination resistors

2. BUS TERMINATION:

Especially for longer busses and/or multiple nodes connected to the bus and/or high communication

speeds, the bus should be properly terminated at both ends. Termination resistor (120 Ohm) maybe

added externally at both ends of the bus or the on-board termination maybe activated by adding a

jumper (see 5).

3. NUMBER OF NODES:

The RS485 electrical interface stadard (EIA-485) allows up to 32 nodes to be connected to a single bus.

The bus transceiver used on the TMCM-3351units (SN65HVD3082ED) has just 1/8th of the standard

bus load and allows a maximum of 256 units to be connected to a single RS485 bus.

4. NO FLOATING BUS LINES:

Avoid ﬂoating bus lines while neither the host/master nor one of the slaves along the bus line

is transmitting data (all bus nodes switched to receive mode). Floating bus lines may lead to

communication errors. In order to ensure valid signals on the bus it is recommended to use a resistor

network connecting both bus lines to GND resp. +5V. In contrast to the termination resistors this

network is normally required just once per bus. Certain RS485 interface converters available for PCs

already include these additional resistors (e.g. USB-2-485).

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