User manual

PD-1161 V1.0 Hardware Manual (Rev. 1.13 / 2012-JUL-30) 10
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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 PD-1161 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 RS-485 electrical interface standard (EIA-485) allows up to 32 nodes to be connected to a
single bus. The bus transceiver used on the PD-1161 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
3.2.2 USB Connector
Pin
Label
Description
1
VBUS
+5V power
2
D-
Data –
3
D+
Data +
4
ID
not connected
5
GND
ground
Table 3.2 Mini USB connector