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Communications Sequence The right to send a frame is called the “transmission right.” The Unit that has the
transmission right is the one that can send a frame at any given time. The trans-
mission right is traded back and forth between the host computer and the PC
each time a frame is transmitted. An example communications sequence be-
tween the host computer and PC is described below.
• The host computer sets a delimiter at the end of the first command frame and
transmits the frame.
• When the PC receives the delimiter, it returns the same delimiter to the host
computer.
• After receiving the delimiter from the PC, the host computer transmits the next
frame.
• The PC sets a delimiter at the end of the first response frame and transmits the
frame.
• When the host computer receives the delimiter, it returns the same delimiter to
the PC.
• After receiving the delimiter from the host computer, the PC transmits the next
frame.
• Long transmissions are managed by exchanging delimiters in this way. The
last frame ends with a terminator.
Delimiter
Text
@ Unit No.
Header code
FCS
Delimiter
Frame1 (command)
Text
FCS
Delimiter
Terminator
FCS
Text
End code
Header code
@ Unit No.
Frame2 (command)
Frame (response)
Delimiter
Text
FCS
Terminator
Frame3 (command)
Host com-
puter
PC
Using the TXD(––) Instruction The TXD(––) instruction can be used to transmit data from the PC’s data area to
the host computer. There is no response from the host computer. The TXD(––)
instruction will be executed after the response has been transmitted if TXD(––)
is executed while a response to a Host Link command is being returned to the
host computer.
No response
Text
@ Unit No.
Header code
FCS
Terminator
Host computer
PC
Host Link Communications
Section 8-2