Omron CQM1H-SCB41 Operation Manual page 111

Using Protocol Macros
Ladder Diagram
96
Disable Mode, it is not sufficient to simply set the Echoback Disable Mode Flags to
0 (OFF) while executing the protocol macro. Either switch once to the Program
mode, and then start running again, or turn the power supply OFF and then ON.
The Echoback Disable Mode Monitor Flag (Port 1: 20406, Port 2: 20407) can be
used to check the Echoback Disable Mode of each port.
Always ON Port 1 Echoback Disable Mode Flag
25313
Port 2 Echoback Disable Mode Flag
Note The following describes the relationship between the RS-485 2-wire connec-
tion and the echoback.
Because the send line and receive line share a single, paired cable in the RS-
485 2-wire connection, the sent data returns to the cable's own receive line
(see figure on following page). This is called "echoback" in this manual.
In the protocol macro function, in order to enable signals to be sent and
received without the user having to specifically define this echoback data as
an expected receive message, the system is designed to produce only a
reception trace, via firmware, on the Serial Commmunications Board, while
ignoring the data. However, in performance, it is sometimes difficult to distin-
guish between proper responses and echoback data responses due to varia-
tions in the echoback data delay, or variations in the processing time of the
Serial Communications Board firmware, and communication errors may result
by mistaking echoback data for an expected receive message.
The reception of unnecessary echoback messages can thus be prevented by
setting the Echoback Disable Mode Flag to 1 (ON).
2-wire connection using the RS-422A/485 port of the CQM1H-SCB41
Serial Commmunications Board
RS signal
SD signal
RD signal
Echoback
2-wire connection using NT-AL001 and the RS-232C port of the CQM1H-
SCB41
Serial Communications
Board
RS signal
SD signal
RD signal
20706
20707
RS-422A/485
port
External
device
RS-232C
NT-AL001
board
Echoback
Section 5-7
RS-485 2-wire connection
External
device
RS-485 2-wire connection
External External
device device