Data Exchange Timing For Link Words - Omron SYSMAC CS1W-SCB41-V1 Operation Manual

Enhanced Protocol Macro Functions
5-6-1

Data Exchange Timing for Link Words

Pre-Ver. 1.2 Units
Unit Ver. 1.2 or Later
Link word specification (a method of specifying the common area for data
exchanged between the CPU Unit and the Board/Unit) is used as a method
for specifying the storage location of send/receive data when executing com-
munications sequences in protocol macro mode.
The link word area is refreshed between the CPU Unit and Board/Unit during
every CPU Unit cycle. The actual refresh timing with the CPU Unit, however,
does not correspond to whenever the I/O is refreshed. I/O refresh is per-
formed when a request is received from the Board/Unit (according to the
send/receive command executed in the communications sequence, and
therefore not synchronized with the CPU Unit scantime). This is called on-
request I/O refreshing.
CPU Unit
Ladder program
PMCR
I/O memory
(2)
IN
Receive data
OUT
Send data
Therefore, when a send command is executed ((1) in the above diagram), the
read processing of data using variables from the I/O memory addresses in the
CPU Unit specified in the link words is performed at the start of I/O refreshing
(2). The send message is compiled based on this data (3), after which the
actual message is sent (4). As a result, the actual message send timing is not
when the send command is executed, but waits until the next I/O refresh with
the CPU Unit.
The refresh between the CPU Unit and Board/Unit is the same as in earlier
models as far as being the CPU Unit's refresh timing. I/O refresh with the CPU
Unit, however, is continuous during execution of the PMCR(260) instruction,
regardless of any request from the Board/Unit. When the send/receive com-
mand is executed, the data from within the Board/Unit is accessed and
exchanged. This method is called continuous I/O refresh and is set in the
Setup Area allocated in the DM Area.
Board/Unit
Communications
sequence
(1)
Step 0
Send message according
Step 1 (send)
to send command
Link word 1
I1
Receive data
O1
Send data
Earlier models refresh I/O when a request is
received (using send/receive commands in the
communications sequence).
Section 5-6
(1) to (4) are performed in
order for the processing within
this dotted line.
(4) Send
R (O1, 2)
Read and compile message
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