Automation Direct D0-DCM, Руководство пользователя

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Installation and
Safety Guidelines
Installation
and Setup
2–7
Installation and Setup
Data Communications Module, 1st Edition, 08/05
RXD+
RXD–
TXD+
TXD–
Signal GND
PORT 2
(D0–DCM)
RS–422 Slave
9 TXD+
10 TXD–
13 RXD+
6
RXD–
11 RTS+
12 RTS–
14 CTS+
15 CTS–
7
0V
PC/PLC Master
Termination
Resistor on
last slave only
D0–DCM Port 2
RS–422 Network
Cable: Use Belden
9729 or equivalent
RS–422/485 Multi-drop Termination Resistors — It is important you add
termination resistors at each end of the RS422/485 line. This helps reduce data
errors during data transmission. You must select resistors that match the cable
impedance. For example, a typical 22 AWG solid conductor cable with 4.5 twists per
foot has a typical impedance of about 120 ohm.
There are two ways to actually connect the resistors.
•
Line-to-Line — this method balances the receive data lines (IN+ and
IN–) and requires one resistor at each end of the line. (The cable
diagrams we’ve provided show this method, but you can use either).
•
Line-to-Ground — this method also balances the receive data lines, but
common mode noise rejection is improved significantly. This method
requires two resistors at each end of the line. Also, since there are two
resistors, the sum total of both resistors must match the cable
impedance.