27 TROUBLESHOOTING
27.3 Troubleshooting by Symptom
469
27
Hardware information communication error "NAK" occurs
Communication is intermittent
Symptom Cause Corrective action MC Non Bi Pd
Hardware information
communication error signal
"NAK" turns ON.
Refer to section which
describes the communication
error signal ("C/N", "P/S",
"PRO", "SIO") that are turned
ON simultaneously when the
"NAK" signal is turned ON.
• Take corrective action corresponding to the contents of the
error.
■Remarks
When the "NAK" signal is turned ON, the error code is added to "NAK" and is transmitted from
C24 to the target device. Take the corrective action depending on the error code listed below.
Page 473 Error Code List
In addition, the error code when "NAK" is transmitted is stored to the C24 buffer memory shown
below, so it can be checked at the C24 side as well.
• Error code storage buffer memory
CH1 side: Un\G602
CH2 side: Un\G618
Symptom Cause Corrective action MC Non Bi Pd
Communications are
intermittently established and
lost.
During multidrop connection,
two or more devices
transmitted data at the same
time.
• Connect the target devices to C24 on a 1:1 basis and
perform a loopback test.
If C24 can communicate normally with each of all target
devices, two or more devices transmit data simultaneously
during multidrop communication. Therefore, interlock the
devices not to transmit at the same time.
The signal cable wiring has a
loose connection.
• Replace the cable, or make the connections firm.
The CD(DCD) signal turns
ON/OFF repeatedly when the
CD terminal check is enabled.
• Change the wiring so that the CD(DCD) signal remains ON
all the time, or change the setting so that the CD terminal
check is disabled.
When half-duplex
communications is set, the
ON/OFF timing of each signal
does not match.
• Control at the target device side so that the ON/OFF timing
of each signal matches.
For details, refer to the following section.
(Page 243 DATA COMMUNICATIONS USING HALF-
DUPLEX COMMUNICATIONS)
The next transmission
processing was requested
before a previous
transmission processing had
been completed.
• Make sure to obtain handshake in the program.
The next reception
processing was requested
before a previous reception
processing had been
completed.
Communication via modem is
disrupted.
• Since communication via modem may be disrupted due to
low line quality or instantaneous interruption, create a
program for reconnecting the line in case of unexpected line
disruption.
• If this symptom occurs frequently, reduce the
communication rate of the modem, and retry the
communication.
The data sent from C24 are
received back to the RS-422/
485 interface (CH2) side.
• Disable the echo back with the Echo back enable/disable
designation before communication.
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