GE GFK-1541B - Manual (Page 117)

4-2 TCP/IP Ethernet Communications for the Series 90™ PLC User's Manual – May 2002 GFK-1541B
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:
ƒ The COMMREQ Function Block (ladder instruction)
ƒ The COMMREQ Command Block
ƒ The Channel Command
ƒ Status Data (COMMREQ Status word, LAN Interface Statusand Channel Status bits)
ƒ The logic program controlling execution of the COMMREQ Function Block
The figure below illustrates the relationship of these elements:
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.
a44916c
INPUTS
AND
OUTPUTS
FOR COMMREQ
FUNCTION
COMMAND
BLOCK
ADDRESS
COMMREQ
FUNCTION BLOCK
DETAILS
OF THE
CHANNEL
COMMAND
COMMREQ
COMMAND BLOCK
COMMREQ
STATUS WORD
STATUS
CODES
COMMREQ
STATUS
WORD
ADDRESS
STATUS BITS
LAN INTERFACE STATUS
AND CHANNEL STATUS
BITS
INITIATES
COMMREQ
FUNCTION
BLOCK
CONTROL
LOGIC
Location in PLC memory
specified when configuring
the Interface using
Configuration Software