Omron Programmable Terminals NT631C - Manual (Page 142)

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Connecting to the Host’s RS-422A/485 Port Section 5-2
Note Before connecting or disconnecting cables between devices, make sure that
the power supply to all of the connected devices (NT631/NT631C, PLC, etc.)
is OFF.
Reference: • The internal circuit of the NT631/NT631C is shown below.
• For details on setting the terminal resistance of the host, refer to Setting
methods for each type of the communications.
5-2-8 Handling the Shield on RS-422A/485 Cables
Perform the following procedures to connect, process the shield, and connect
to ground for communications systems using the RS-422A/485. Incorrect con-
nection may result in communications errors with the host.
Connecting the Ground Wire
The PT has a functional ground terminal (FG: ).
1,2,3... 1. Ground according to Figure (a) for normal grounding.
• Connect the ground terminal (GR) of the devices to the functional ground
(FG: ).
Make sure that each signal line is grounded at only one point and ground
to 100
Ω max.
• Short the LG terminal of the PLC to the ground terminal (GR).
• Use a wire gauge of at least 2mm
2
for the ground wire.
• Refer to the manual for individual Communications Units for details on
proper wiring procedures.
2. Do not ground the functional ground (FG: ) of the PT if it is mounted to
the same panel as devices that generate noise, such as motors or invert-
ers, as shown in Figure (b).
Preparing the Shield of RS-422A/485 Connectors
Always prepare RS-422A/485 cable shields properly. Otherwise, communica-
tions errors may occur with the host.
Ground only one end of the shield when connecting the ground terminal (GR)
of the devices to the functional ground (FG: ), and grounding each signal
line at only one point and to 100
Ω max, as shown in Figure (a).
Ground both ends of the shield when not grounding the functional ground
(FG: ) of the PT, as shown in Figure (b).
TRM-RDA
terminals
Function
Shorted Terminal resistance is applied.
Short only at the NT631/NT631C connected to the end of an
RS-422A/485 cable.
Open Terminal resistance is not applied.
Leave the terminals open when connecting an NT631/NT631C
anywhere other than at the end of the RS-422A/485 cable.
Terminal resistor (120 Ω)
RDB
RDA
TRM
+
-
Making a connection here inserts a terminator between
+ (RDB) and - (RDA).