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has received the Xon code from the host computer (refer to (2) in the above tim-
ing chart).
(2)
(1)
Data transmission from Host Computer
Host computer
Host Link Unit
Data
Xoff Xon
Succeeding data
When the host computer receives an Xoff code from the Host Link Unit, the host
computer stops transmitting data (refer to the above (1)) and when the host com-
puter receives an Xon code from the Host Link Unit, the host computer resumes
transmitting the data. Each communications port of the Host Link Unit incorpo-
rates eight buffers used for the transmission and reception of data. If seven of
the eight buffers are in use, the Xoff code is transmitted and if two of the eight
buffers are available, the Xon code is transmitted. If the Host Link Unit needs to
transmit the Xon or Xoff code while the Host Link Unit is transmitting data, the
data transmission is interrupted to send the code. After the code is transmitted,
the Host Link Unit resumes transmitting the data.
If the Host Link Unit is connected to the host computer via an RS-232C interface,
half-duplex communications are possible by selecting half-duplex communica-
tions using the CPU Bus Unit System Setup. In half-duplex communications, CD
signals are checked and the reception of data will take precedence. In half-du-
plex communications, the following two settings are possible.
Retry Setting Any data interrupted during transmission can be re-transmitted from the begin-
ning or transmitted from the interrupted part.
It is possible to adjust the time between turning ON the RS signal and the start of
data transmission from 0 to 510 ms in 2-ms increments.
The following timing chart shows the timing of half-duplex communications.
Data A
Succeeding por-
tion of data A
Data B
(1)
(2) (3) (4)
(5)
Host computer
Host Link Unit
Output direction
*1 *1
*2
Note 1. The abbreviations of the signals names are as follows:
SD: send data, RS: request to send, CS: clear to send, CD: carrier detect,
RD: receive data
2. Signal directions are in reference to the Host Link Unit.
3. *1 is the transmission delay time.
Timing of Half-duplex
Communication
Transmission Delay Time
Setting
Communications Timing Section 4-5
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