Table of Contents
Advertisement
3.2 Terminal Arrangement and Signal Names and the Connection Procedure of the RS-422A/485 Communication Interface
Host
computer
Terminator (externally attached) 120 Ω, 1/2 W or more
RS-232
RD( – )
(A)
RD( + )
(B)
TD( – )
TD( + )
SHIELD
(SG)
Converter
Note
• Serial Interface Converter
(The following figure illustrates the case when the host computer's interface is RS-232.)
RS-422A/485
terminal on the
Recorder
SD A
SD B
RD A
RD B
SG
(SG)
FG
#1
Do not connect terminator to #1 to #n–1
•
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.
Some converters 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.
Terminator (externally attached)
SD A
(A)
SD B
(B)
RD A
RD B
SG
(SG)
FG
#2
CAUTION
SD A
(A)
SD B
(B)
RD A
RD B
SG
FG
#n
(#n ≤ 31)
3-5
3
Advertisement
Table of Contents
