LINK Systems OmniLink - PLC Interface - Manual (Page 31)

June 5, 2000 Man ual Rev 1 .0
5.4
Section 5.6 Requests From the PLC to Link Equipment
The PLC can request certain information from the Link equipment. The commands that do this differ
from regular commands in that they cause the Link equipment to not only generate a reply to the PLC,
but expect the PLC to echo the reply back. This was done to ensure reliable data transfer. When the
Link equipment gets an information request command, it will send the information and expect the
command to be echoed back from the PLC for verification. If the verification fails, there will be the
usual 2 retries. If the retries fail, a communications error will be displayed. For example, if a PLC
wants to query the value of user parameter 3 the sequence might be:
From PLC - [LF]U003?[CR]
(Request for value of U03)
From Link - [LF]U003=000012345[CR]
(Value of U03)
From PLC - [LF]U003=000012345[CR]
(Echo back for verification)
This simplifies programming the PLC in that the last two messages are exactly the same as generated by
the Link equipment when the user changes a parameter value or a job is recalled.
Section 5.6.1 Request for Job Number
From PLC:
[LF]JOB?[CR]
Reply from Link:
Generates Job Number Command (See Section 5.5.1).
Notes:
None.