68
Link Unit completes transmitting the response. If no response from the host
computer is required, the execution possible flag will turn ON when the com-
mand to the host computer is delivered from the PC to the Host Link Unit.
The following timing charts show the transmission timing in half-duplex commu-
nications in which the host computer is waiting to send a response after the host
computer transmitted data.
(2)
Command
Command
1
0
Command
Command
Response required
Host computer
Host Link Unit
Response
Response
Execution possible flag
Response not required
Host Link Unit
Execution possible flag
Reception of command
End of reception
Response
Host computer
(2)
(1)
(1)
Reception of command End of reception
1
0
Response delay
Response delay
If the command format sent from the host computer includes the response delay
setting, the Host Link Unit will not send a command to the host computer until the
response delay time passes (refer to (1) of the above). A response for a com-
mand from the host computer will be transmitted only after the Host Link Unit
completes transmitting the command to the host computer (refer to (2) of the
above). If no response from the host computer is required, the execution pos-
sible flag will turn ON when the command to the host computer is delivered from
the PC to the Host Link Unit.
Xon/Xoff Control Timing Full-duplex communication between the host computer and PC is possible us-
ing the Xoff (transmission interrupt) code and Xon (transmission interrupt can-
cel) code, with which it is possible to control the transmission and reception of
data. The default value of the Xoff code is 13 and that of the Xon code is 11 (both
hexadecimal). The values can be changed using the CPU Bus Unit System Set-
up. No Xon or Xoff code will be retrieved as reception data. The following timing
charts show Xon and Xoff control timing.
(1)
(2)
Data transmitted from Host Link Unit
Host computer
Host Link Unit
Data
Xoff Xon
Succeeding data
The Host Link Unit stops transmitting the data when the Host Link Unit has re-
ceived the Xoff code from the host computer (refer to (1) in the above timing
chart). The Host Link Unit starts transmitting the data when the Host Link Unit
Communications Timing Section 4-5
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