TMCM-1161 V1.0 Hardware Manual (Rev. 1.14 / 2013-JUL-23)

9

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3.2.1.2 RS485

For remote control and communication with a host system the PD-1161 provides 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

n - 1

node

n

Host

Slave

Slave

Slave

RS485

termination

resistor

(120 Ohm)

termination

resistor

(120 Ohm)

}

keep distance as

short as possible

Figure 3.5: RS485 bus structure

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. The PD-1161 does

not integrate any termination resistor. Therefore, 120 Ohm termination resistors at both ends of

the bus have to be added externally.

3. NUMBER OF NODES:

The RS-485 electrical interface standard (EIA-485) allows up to 32 nodes to be connected to a

single bus. The bus transceiver used on the PD-1161 units (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 as well defined logic levels. 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).

node

n - 1

node

n

Slave

Slave

termination

resistor

(120 Ohm)

+5V

GND

pull-up (1k)

pull-down (1k)

RS485- / RS485B

RS485+ / RS485A

Figure 3.6: RS485 bus lines with resistor network