_________________________Shihlin Electric SPM-3 User Guide__________________
19
Chapter 5 Communication
5.1 RS485
5.1.1 RS485 standard
PARAMETERS
Mode of Operation
Differential
Number of Drivers and Receives
32 Drivers / 32 Receivers
Maximum cable length( meters)
1200
Maximum data rate ( baud )
10M
Maximum common mode voltage ( Volts )
12 to -7
Maximum Driver Output Levels ( Loaded )
+/- 1.5
Maximum Driver Output Levels ( Unloaded )
+/- 6
Drive Load ( Ohms)
60( min )
Driver Output short circuit Resistance ( kohms) 150 to Gnd, 250 to -7 or 12V
Minimum receiver input Resistance ( kohms)
12
Receiver sensitivity
+/- 200mv
5.1.2 Wiring for instruments communication
RS485 communication must use twisted paired wire, as shown in the following program. “D+” connects
to one wire and “D-” to the other one. There should be one 120 ohm resistor at each end of the
communication wire to balance impedance.
Cautions :
There must be no more than two wires connected to each terminal, this ensures that
a “Daisy Chain” or “Straight Line” configuration is used. A “star” or a network with
“Stubs(Tees)” is not recommended as reflections within the cable may result in data
corruption
。
Master
D- D+
D- D+
D- D+
D- D+
120 ohm
120 ohm
Slave
Slave
Slave