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Chapter 7. Programming SRTP Channel Commands
GFK-2224Q January 2017 121
(Word 9) Number of Write Repetitions: Word 9 specifies the number of writes to be performed before
automatically completing the communications request and closing the channel. If this value is set to 1, only a
single write will be issued. If this value is set to 0, writes will be issued on the requested period until the channel
is aborted.
(Word 10) Time Units for Write Period: Words 10–11 together define how often the write is to be performed
(
write period
). Word 10 specifies the time unit such as seconds or minutes for the write period. Word 11
specifies the number of those units. The choices for the time units are:
Value Meaning
1 hundredths of seconds (10ms)
2 tenths of seconds (100ms)
3 seconds
4 minutes
5 hours
(Word 11) Number of Time Units for Write Period: Word 11 specifies the number of time units for the write
period. The write period is in effect even when the Channel command is setup to issue a single write.
Example Write Period Calculation: If Word 10 contains a value of 3 specifying seconds as the time unit and
Word 11 contains a value of 20, then the write period is 20 seconds.
A Channel command set up to issue a single write can have only one pending write transfer. A write will normally
be issued at the start of each write period. If the pending write transfer has not completed during the write
period, the Channel Error bit and Detailed Channel Status words will be set to indicate a non-fatal period error.
If the period error occurs on the first transfer, the COMMREQ Status will also indicate a non-fatal period error.
Note: The COMMREQ Status is issued only once for each COMMREQ; for more information, see “Using
COMMREQs for Channel Commands”. The pending transfer can still complete after the period error occurs.
You can determine when the pending transfer completes by monitoring the Channel Error and Data Transfer
bits. For Channel commands set up to issue multiple writes, the next write transfer will be issued only after the
pending write transfer completes
.
If the Number of Time Units is zero, a subsequent transfer will be issued as soon as the previous transfer
completes. In this case, no period errors are reported by the Channel Error bit.
(Word 12) Timeout for Each Write: Word 12 specifies the time (in hundredths of a second) the Ethernet
Interface will wait for a write transfer to complete before setting the Channel Error bit and Detailed Channel
Status bits to indicate a non-fatal timeout error. If the timeout error occurs on the first transfer, the COMMREQ
Status (will also indicate a non-fatal timeout error. Note: The COMMREQ Status is issued only once for each
COMMREQ; for more information, see “Using COMMREQs for Channel Commands”. The transfer can still
complete even after a timeout occurs. You can determine when the pending transfer completes by monitoring
the Channel Error and Data Transfer bits. As a result, an application can choose what to do if one occurs. If
the timeout value is specified as zero, no timeout errors will be reported.
For most applications a timeout is not needed because the write period acts as a timeout. (Word 12 should be
zero for no timeout.) However, there are two special circumstances in which specifying a timeout is
recommended:
▪
When the number of time units (Word 11) is zero, so that a subsequent transfer will be issued as soon as
the previous transfer completes and no period errors are reported. In this case a timeout value can be
specified so that the Channel Error bit will report timeout errors.
▪
When the write period is very long (minutes or hours). In this case a shorter timeout value can be specified
so the application doesn’t have to wait for the write period to expire before taking action.