User manual

TMCM-1021 V1.2 Hardware Manual (Ref. 1.00 / 2012-MAR-09) 10
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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-1021 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 to 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: Bus lines with resistor network
3.5 Inputs and Outputs
3.5.1 Multi-purpose Inputs
The eight pin connector of the TMCM-1021 provides four general purpose inputs IN_0, IN_1, IN_2 and IN_3.
The first two inputs have dedicated connector pins while the other two share pins with two general
purpose outputs.
All four inputs are protected using voltage resistor dividers together with limiting diodes against voltages
below 0V (GND) and above +3.3V DC (see figure below).
+3.3V
IN_0
IN_1
IN_2
IN_3
microcontroller
and TMC262
10k
10k
1nF
GND
GND GND
Figure 3.7: General purpose inputs