User Manual

TMCM-1311 V1.2 Hardware Manual (Rev. 1.12 / 2013-JUL-05) 15
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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-1311 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 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 3.5 RS485 bus lines with resistor network