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10. COMMUNICATIONS IN THE BIDIRECTIONAL MODE
MELSEC-A
(5)
NAKcode
(a) Transmitting NAK from an AJ71C24 t o a computer
The NAK response is given from an AJ71C24 to a computer if an
error is detected.
Therefore, the NAK response might be given while the computer is
transmitting data if communications is made in the full-duplex
mode.
I
An AJ71C24 ignores the designated length of received data if it
detects an error while receiving data. If the data length is incor-
rect, the data received is ignored until the ENQ code is received.
I
(b) Transmitting NAK from a computer to an AJ71C24
To transmit the NAK from a computer to an AJ71C24, transmit a
2-byte error code following the NAK code.
If the NAK code is received as the response, execute error processing
according t o the error code received directly after the NAK code.
The error codes related to the bidirectional mode communications are
described in Section 11.2.
(6)
Time-out check by a computer
If a time-out check is made for data transmitted from a computer send
area to an AJ71C24 receive area in the bidirectional mode, the time-out
check time to be set must. be longer than the value shown below.
(Maximum scan time of the PC CPU x
2)
+
100 msec
(7)
NULL code send from an AJ71 C24
A framing error might occur at the AJ71C24 if nothing is transmitted
from a computer t o an AJ71C24 via the RS-422 interface. In this case,
the AJ71C24 sends ' O OH' (NULL code) to the computer receive area.
These NULL codes should be ignored by the computer.
The computer should also ignore all data transmitted from the AJ71C24
prior t o an ENQ, ACK, or NAK code.
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