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Host Link Communications Section 4-1
4-1 Host Link Communications
Host Link System can be used to send C-mode commands or FINS com-
mands from a host computer (e.g., personal computer or PT) to a PLC to
read/write I/O memory, control operating modes, etc. The PLC can also use
the SEND(090), RECV(098), and CMND(490) instructions to send FINS com-
mands to the host computer for “slave-initiated communications.” Using the
PLC to control communications enables controlling and monitoring of the
overall system operating status.
Refer to the CS/CJ-series Communications Commands Reference Manual
(W342) for details on C-mode and FINS commands.
Host-initiated
Communications
Communications in a Host Link System are normally started by the host com-
puter.
The host computer sends a command to the PLC. The PLC processes the
command and returns a response to the host computer. This process is
repeated, allowing the host computer to monitor and control PLC operation.
Both C-mode and FINS commands can be used in host-initiated communica-
tions.
PLC-initiated
Communications
The PLC can also initiate communications with the host computer, as may be
necessary when errors occur on the line controlled by the PLC or to confirm
the operating status of the host computer.
The PLC sends a command to the host computer through a Serial Communi-
cations Unit or Board. The host computer processes the command and, when
necessary, returns a response to the PLC.
Using PLC-initiated communications requires that a program be prepared on
the host computer to process commands received from a PLC and return the
required responses. Only FINS commands can be used in PLC-initiated com-
munications.
Note Always transfer uppercase letters when using Host Link communications.
Lowercase letters cannot be processed.
4-2 Setup Area Allocations
This section describes the Setup Area allocated to the Serial Communications
Board and Serial Communications Units in the DM Area when Host Link com-
munications are used.
4-2-1 Setup Area Words
The Serial Communications Board and Serial Communications Units use the
following words as a Setup Area in the DM Area when Host Link communica-
tions are used. The words allocated to the Serial Communications Board are
different from those allocated to the Serial Communications Units (which are
allocated words according to the unit numbers).
Host computer
CS/CJ-series PLC
Command
Response
Host computer
CS/CJ-series PLC
Command
Response
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