User manual
TMCM-1110 stepRocker Hardware Manual (Ref. 1.08 / 2012-APR-04) 12
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3.5 Communication
3.5.1 RS485
For remote control and communication with a host system the TMCM-1110 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 TMCM-1110 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 transceiver used on the TMCM-1110 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