TMCM-3351 Hardware Manual • Hardware Version V1.00 | Document Revision V1.00 • 2017-SEP-01
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7 Communication
7.1 RS485
For remote control and communication with a host system the TMCM-3351 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 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
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-3351 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 communi-
cation 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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