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TCP/IP Ethernet Communications User’s Manual - August 1997 GFK-1084B
The SYSID field of the COMMREQ Function Block defines the target rack and slot of
the Ethernet Interface to receive the command data. The first two digits of SYSID
(00 in this example) indicate the rack number, the last two digits (04 in this example)
indicate the slot number of the Ethernet Interface.
The TASK field of the COMMREQ Function Block indicates which mailbox task ID to
use for the specified rack and slot. This field should always be zero (0) for the
The FT output (CMRQFLT in this example) is turned on (set to 1) if there were
problems preventing the delivery of the Command Block to the Ethernet Interface.
In this case, the other status indicators are not updated for this COMMREQ.
Troubleshooting Your Ladder Program
As explained in Section 4 of this chapter, there are several forms of status data which can
be used in your ladder program. The use of the LAN Interface OK bit in the LAN
Interface Status Word was described in the ladder program fragment above. Some of
the status data can be used to troubleshoot your program in its developmental stage.
The two primary sources of this data are the FT Output on the COMMREQ Function
Block and the COMMREQ Status word (CRS word).
FT Output is ON
If after executing a COMMREQ Function, the FT Output is ON, then there is a
programming error in one or more of the following areas.
Invalid rack/slot specified. The module at this rack/slot is unable to receive a
COMMREQ Command Block.
Task ID not valid. (Should be set to 0.)
Data Block length is specified as 0 or greater than 128.
COMMREQ Status Word is Zero (0) and FT Output is OFF
If after executing a COMMREQ function, the CRS word is zero (0) and the FT Output is
OFF, then the Command Block has been sent to the Ethernet Interface, but no status has
been returned yet. If this condition persists, check the PLC fault table for information.
COMMREQ Status Word is Not One (1)
If after executing a COMMREQ function, the CRS word is not one (1) indicating success,
then there were:
Errors in the Command Block (the Channel Command code or parameters), or
For an Establish Read or Write Channel Command, the command parameters were
valid but there was an error in establishing a channel.
If the CRS word does not contain a 1 indicating success, then it contains either a 0 or a
code indicating what error occurred. See Section 4 of this chapter for CRS word error