3-5
IM 04P01B01-17E
3
Using the RS-422A/485 Communication Interface (/C3 Option)
(The following figure 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 terminator to #1 to #n–1
#2 #n
(#n ≤ 31)
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
RS-422A/485
terminal on the
Recorder
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 recorder’s ground (one-sided
grounding). This is effective when there is a difference in the electric potential between the
computer’s ground and the recorder’s ground. This may be the case for long distance
communications. If there is no difference in the electric potential between the computer’s
ground and the recorder’s ground, the method of connecting the shield also to the 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 type 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.
• Serial Interface Converter
Recommended converter
MODEL RC-57 by RA SYSTEMS CORP.
CAUTION
Some converters not recommended by Yokogawa have FG and SG pins that
are not isolated. In this case, do not connect anything to the converter’s FG and
SG pins (unlike the figure on the previous page). Especially in the case of long
distance communications, the potential difference that appears may damage the
recorder or cause communication errors. For converters that do not have the SG
pin, they can be used without using the signal ground. For details, see the
manual that came with the converter.
On some non-recommended converters, the signal polarity may be reversed (A/B or
+/- indication). In this case, reverse the connection.
3.2
Terminal Arrangement and Signal Names and the Connection Procedure of the RS-422A/485 Communication Interface
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