TMCM-1260 Hardware Manual • Hardware Version V1.10 | Document Revision V1.20 • 2018-05-17
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8 Communication
8.1 RS485
For remote control and communication with a host system the TMCM-1260 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 10: 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. The TMCM-1260 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 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-1260 units (SN65HVD1781D) offers a significantly reduced
bus load compared to the standard and allows a maximum of 255 units to be connected to a single
RS485 bus using standard TMCL firmware.
Please note: usually it cannot be expected to get reliable
communication with the maximum number of nodes connected to one bus and maximum supported
communication speed at the same time. Instead, a compromise has to be found between bus cable length,
communication speed and number of nodes.
4.
COMMUNICATION SPEED:
The maximum RS485 communication speed supported by the TMCM-1260 hardware is 1Mbit/s.
Factory default is 9600 bit/s. Please see separate TMCM-1260 TMCL firmware manual for information
regarding other possible communication speeds below the upper hardware limit.
5.
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.
There are actually two options which can be recommended: Add resistor (bias) network on one side
of the bus, only (120R termination resistor still at both ends):
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