Yokogawa Data Acquisition with PID Control CX2000 - Manual (Page 87)

3-7
IM 04L31A01-17E
Serial Interface
3
Two-Wire System
Connect the transmission and reception signals with the same polarity on the RS-422/485
terminal block. The two-wire cable can be used only when using the Modbus protocol.
Connect terminators (120 Ω 1/2 W or more) to the signal line of the host computer and
the terminal at the of the chain.
Terminator (externally attached) 120 Ω, 1/2 W or more
#1
Do not connect terminators to #1 through #n-1.
RS-422/485
pins on the
CX
#2 #n
(#n ≤ 32)
Terminator (externally attached)
Host
computer
SG
RDB( + )
RDA( - )
SDB( + )
SDA( - )
FG
SG
RD B
RD A
SD B
SD A
(SG)
(B)
(A)
(B)
(A)
FG
SG
RD B
RD A
SD B
SD A
(SG)
FG
SG
RD B
RD A
SD B
SD A
(SG)
(B)
(A)
The following diagram illustrates the case when the host computer’s interface is RS-232.
Terminator (externally attached) 120 Ω, 1/2 W or more
#1
Do not connect terminators to #1 through #n-1.
RS-422/485
pins on the
CX
#2 #n
(#n ≤ 32)
Terminator (externally attached)
FG
SG
RD B
RD A
SD B
SD A
(SG)
(B)
(A)
(B)
(A)
FG
SG
RD B
RD A
SD B
SD A
(SG)
FG
SG
RD B
RD A
SD B
SD A
(SG)
(B)
(A)
SHIELD
TD( + )
TD( - )
RD( + )
RD( - )
Host
computer
Converter
RS-232
Note
• The method used to eliminate noise varies depending on the situation. In the connection
example, the shield of the cable is connected only to the FG terminal (ground) (one-sided
grounding). This is effective when there is a difference in the electric potential between the
host computer’s ground and the CX’s ground. This may be the case for long distance
communications. If there is no difference in the electric potential between the host computer’s
ground and the CX’s ground, the method of connecting the shield also to the host computer’s
ground may be effective (two-sided grounding). In addition, in some cases, using two-sided
grounding with a capacitor connected in series on one side is effective. Consider these
possibilities to eliminate noise.
• When using the two-wire interface (Modbus protocol), the 485 driver must be set to high
impedance within 3.5 characters after the last data byte is sent by the host computer.
• There is no problem of connecting a 220-Ω terminator at either end if YOKOGAWA’s PLCs or
temperature controllers are also connected to the communication line.
3.3 Terminal Arrangement and Signal Names and the Connection Procedure of the RS-422/485 Interface