GE GFK-1541B manual Communications Request

4

The Communications Request

“Communications Request” is a term used to describe all the user elements required for correctly initiating Channel commands from a Series 90 PLC. This section describes the elements of the Communications Request.

No programming of Communications Requests is required for PLCs acting as servers, which are merely targets of other systems’ requests, but do not themselves initiate requests.

Structure of the Communications Request

The Communications Request is made up of the following elements:

ƒThe COMMREQ Function Block (ladder instruction)

ƒThe COMMREQ Command Block

ƒThe Channel Command

ƒStatus Data (COMMREQ Status word, LAN Interface Statusand Channel Status bits)

ƒThe logic program controlling execution of the COMMREQ Function Block

The figure below illustrates the relationship of these elements:

CONTROL

LOGIC

INITIATES COMMREQ FUNCTION BLOCK

COMMREQ

FUNCTION BLOCK

INPUTS

AND

OUTPUTS

FOR COMMREQ

FUNCTION

COMMAND

BLOCK

ADDRESS

COMMREQ

COMMAND BLOCK

COMMREQ

STATUS

WORD

ADDRESS

DETAILS

OF THE

CHANNEL

COMMAND

a44916c

COMMREQ

STATUS WORD

STATUS

CODES

STATUS BITS

LAN INTERFACE STATUS AND CHANNEL STATUS BITS

Location in PLC memory specified when configuring the Interface using Configuration Software

COMMREQ Function Block

The COMMREQ Function Block is the ladder instruction that triggers the execution of the Channel command. In the COMMREQ Function Block, you specify the rack and slot location of the Ethernet Interface, a task value, and the address of a location in memory that contains the Command Block. There is also a fault output on the COMMREQ Function Block that indicates certain programming errors.

4-2 TCP/IP Ethernet Communications for the Series 90™ PLC User's Manual– May 2002 GFK-1541B