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Communications Procedure
Section 6-3
6-3
Communications Procedure
Host Link communications are executed by means of an exchange of com-
mands and responses between the host computer and the PC.
With the CQM1H, there are two communications methods that can be used.
One is the normal method, in which commands are issued from the host com-
puter to the PC. The other method allows commands to be issued from the
PC to the host computer.
Frame Transmission and Reception
Commands and responses are exchanged in the order shown in the illustra-
tion below. The block of data transferred in a single transmission is called a
“frame.” A single frame is configured of a maximum of 131 characters of data.
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
transmission right is traded back and forth between the host computer and the
PC each time a frame is transmitted. The transmission right is passed from
the transmitting Unit to the receiving Unit when either a terminator (the code
that marks the end of a command or response) or a delimiter (the code that
sets frames apart) is received.
Commands from Host
In Host Link communications, the host computer ordinarily has the transmis-
sion right first and initiates the communications. The PC then automatically
sends a response.
Commands from PC
With CQM1H PCs, it is also possible in Host Link communications for the PC
to send commands to the host computer. In this case it is the PC that has the
transmission right and initiates the communications.
Terminator
FCS
Text
Text
End code
Header code
Unit no.
Unit no.
Header code
FCS
Terminator
Frame (response)
Frame (command)
Next frame transmission
enabled (i.e., transmission
right transferred)
Text
Unit no.
Header code
FCS
Terminator
Frame (command)
Terminator
FCS
Text
End code
Header code
Unit no.
Frame (response)
Host
computer
PC
No response
Text
Unit no.
Header code
FCS
Terminator
Host
computer
PC
