3

The Communications Request

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

Elements of the Communications Request

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

3-2

TCP/IP Ethernet Communications for the Series 90™ PLC User's Manual– May 2002

GFK-1541B

Page 69
Image 69
GE GFK-1541B manual Structure of the Communications Request, Elements of the Communications Request