GE GFK-1541B - Manual (Page 71)

3-4 TCP/IP Ethernet Communications for the Series 90™ PLC User's Manual – May 2002 GFK-1541B
3
Status Data
There are several types of status data available to the client PLC logic program.
LAN Interface Status Bits (LIS Bits):
The LIS bits comprise bits 1–16 of the 80-bit status area. The
location of this 80-bit status area is assigned using the Configuration software in the “Status
Address” field. The LIS bits contain information on the status of the Local Area Network (LAN)
and the Ethernet Interface itself.
Channel Status Bits:
The Channel Status bits comprise bits 17–80 (64 bits) of the 80-bit status
area. When used for SRTP channels, these bits consist of an error bit and a data transfer bit for
each of the channels that can be established (32 channels for the Series 90-70 Ethernet Interface, 16
channels for the Series 90-30 Ethernet Interfaces). Status bits for unused channels are always set to
zero.
COMMREQ Status Word (CRS Word):
The 16-bit CRS word will receive the initial status of the
communication request. The location of the CRS word is assigned for each COMMREQ function
in the COMMREQ Command Block.
Detailed Channel Status Words (DCS Words):
This detailed status data is retrieved for a
particular channel using the Retrieve Detailed Channel Status Command.
FT Output of the COMMREQ Function Block:
This output indicates that the PLC CPU detected
errors in the COMMREQ Function Block and/or Command Block and did not pass the Command
Block to the Ethernet Interface.
The Logic Program Controlling Execution of the COMMREQ Function Block
Care must be taken in developing the logic that controls the execution of the COMMREQ function.
The COMMREQ function must be initiated by a one-shot to prevent the COMMREQ from being
executed repeatedly each CPU scan, as this will overrun the capability of the Ethernet Interface and
possibly require a manual restart. Checking certain status bits before initiating a COMMREQ
function is also important. In particular, the LAN Interface OK bit should be used as an interlock to
prevent execution of the COMMREQ function when the Ethernet Interface is not operational.
Following initiation of a COMMREQ on a channel, no further COMMREQs should be issued to
that channel until a non-zero CRS word has been returned to the program from the Ethernet
Interface.