User Manual

TMCM-1140 V1.3 Hardware Manual (Rev. 1.04 / 2015-JAN-05) 9
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3.3.1.2 RS485
For remote control and communication with a host system the TMCM-1140 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 3.3: 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 TMCM-1140
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 standard (EIA-485) allows up to 32 nodes to be connected to a
single bus. The bus transceivers used on the TMCM-1140 units (hardware V1.2: SN65HVD3082ED,
since hardware V1.3: SN65HVD1781D) have a significantly reduced bus load and allow a
maximum of 255 units to be connected to a single RS485 bus using 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-1140 hardware V1.2 is 115200
bit/s and 1Mbit/s since hardware V1.3. Factory default is 9600 bit/s. Please see separate TMCM-
1140 TMCL firmware manual for information regarding other possible communication speeds
below the upper limit in hardware.
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):