Handling The Shield On Rs-422A/485 Cables - Omron Programmable Terminals NT631 Setup Manual

Connecting to the Host's RS-422A/485 Port
Note
Reference:
5-2-8

Handling the Shield on RS-422A/485 Cables

1,2,3...
TRM-RDA
terminals
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.
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.
• The internal circuit of the NT631/NT631C is shown below.
-
Terminal resistor (120 Ω)
+
• For details on setting the terminal resistance of the host, refer to Setting
methods for each type of the communications.
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. 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
• Refer to the manual for individual Communications Units for details on
proper wiring procedures.
2. Do not ground the functional ground (FG:
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:
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).
Function
RDA
Making a connection here inserts a terminator between
+ (RDB) and - (RDA).
TRM
RDB
).
2
for the ground wire.
) of the PT if it is mounted to
), and grounding each signal
Section 5-2
125