Explanation Of Operation When Using A Computer Link - NAiS FP Series User Manual

Communication Function 1 Computer Link
8.1.2

Explanation of Operation when Using a Computer Link

Command and Response
Instructions pertaining to the PLC are called "commands". These should be issued by
the computer, to the PLC.
Messages sent back to the computer from the PLC are called "responses". When the
PLC receives a command, it processes the command regardless of the sequence
program, and sends a response back to the computer. The computer uses the response
to confirm the results of the command being executed.
MEWTOCOL - COM sketch
Communication is carried out in a conversational format, based on the MEWTOCOL -
COM communication procedures.
Data is sent in ASCII format.
The computer has the first right of transmission.
The right of transmission shifts back and forth between the computer and PLC each
time a message is sent.
Transmission program
Command message
Start Destina- Text
code tion
1 2 3
Command messages should
contain the following content
when sent:
The unit number of the PLC
1
to which the command is
being sent
The type of command
2
Any settings and data re-
3
quired in order to execute
the command
The command and data are
sent to the PLC with the
specified unit number.
Figure 141: FPΣ MEWTOCOL- COM
8 - 4
Computer
Check End
code code
FPΣ
Reception processing program
Command messages sent from
the PLC should contain the follow-
ing content:
The unit number of the PLC that
4
sent the response
Confirmation of whether or not the
5
processing was carried out suc-
cessfully
The type of command processed
6
If the command was used to read
7
data, the data that was read
If an error occurred and the com-
8
mand could not be processed suc-
cessfully, the content of the error
A response is returned, and proc-
essing is carried out based on the
content of the response, such as
retrieving any data that was sent.
Start Source Text
code
4 5 6 7 8
FPΣ
Response message
Check End
code code