User manual

PDx-113-57/60-SE / TMCM-113-57/60-SE Manual (V1.30 / 2009-OCT-28) 14

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 PDx-113-

57/60-SE-485 integrates a 120 Ohm termination resistor that can be placed between both RS485 bus

wires by setting a jumper:

Set jumper for RS485

bus termination

Figure 4.8: Bus termination

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 for the PDx-113-57/60-SE-485 units

(SN65HVD3082ED) has just 1/8 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 well defined logic levels. In

contrast to the termination resistors this network is normally required just once for a bus. Certain

RS485 interface converters available for a PC already include these additional resistors (e.g. USB-2-

485 [USB-2-485]).

node

n - 1

node

Slave Slave

termination

resistor

(120 Ohm)

+5V

GND

pull-up (1k)

pull-down (1k)

RS485- / RS485B

RS485+ / RS485A

Figure 4.9: Resistor network to avoid floating bus lines