User Manual

PD28-1-1021 / PD28-3-1021 V1.4 Hardware Manual (Rev. 1.03 / 2018-JAN-25) 13
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4. COMMUNICATION SPEED:
The maximum RS485 communication speed supported by the PD28-1-1021 / PD28-3-1021 is 115200
bit/s (FW version 1.29 with TMCM-1021 hardware version 1.2 and 1.4). Factory default is 9600 bit/s.
Please see separate TMCM-1021 TMCL firmware manual for information regarding other possible
communication speeds below 115200 bit/s.
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):
node
n - 1
node
n
Slave Slave
termination
resistor
(120R)
+5V
GND
pull-up (680R)
pull-down (680R)
RS485- / RS485B
termination
resistor
(220R)
RS485+ / RS485A
Figure 3.5: Bus lines with resistor (Bias) network on one side, only
Or add resistor (Bias) network at both ends of the bus (like Profibus™ termination):
node
n - 1
node
n
Slave Slave
termination
resistor
(220R)
+5V
GND
pull-up (390R)
pull-down (390R)
RS485- / RS485B
RS485+ / RS485A
termination
resistor
(220R)
+5V
GND
pull-up (390R)
pull-down (390R)
Figure 3.6: Bus lines with resistor (Bias) network at both ends
Certain RS485 interface converters available for PCs already include these additional resistors (e.g.
USB-2-485 with bias network at one end of the bus).
3.4.1.3 Digital Inputs IN_0 and IN_1
The eight pin connector of the PD28-1-1021 / PD28-3-1021 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.