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Host link communications are executed by means an exchange of commands
and responses between the host computer and the PC. With the C200HS, there
are two communications methods that can be used. One is the normal method,
in which commands are issued from the host computer 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 illustration
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 trans-
mission 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 transmission
right first and initiates the communications. The PC then automatically sends a
response.
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
Commands from PC 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.
No response
Text
Unit no.
Header code
FCS
Terminator
Host
computer
PC
Communications Procedure Section 11-1