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Chapter 7. Programming SRTP Channel Commands
GFK-2224Q January 2017 117
(Word 9) Number of Read Repetitions: Word 9 specifies the number of reads to be performed before
automatically completing the communications request and closing the channel. If this value is set to 1, only a
single read will be issued. If this value is set to 0, reads will be issued continuously on the requested period until
the channel is aborted.
(Word 10) Time Unit for Read Period: Words 10–11 together define how often the read is to be performed
(
read period
). Word 10 specifies the time unit such as seconds or minutes for the read period. Word 11
specifies the number of those units. The choices for the time units are shown below.
Value Meaning
1 hundredths of seconds (10ms)
2 tenths of seconds (100ms)
3 seconds
4 minutes
5 hours
Note: If Time Unit Value is 5 (hours), then the maximum usable value of Number of Time Units is 5965.
(Word 11) Number of Time Units for Read Period: Word 11 specifies the number of time units for the read
period. The read period is in effect even when the Channel command is setup to issue a single read.
Example Read 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 read period is 20 seconds.
A Channel command set up to issue a single read can have only one
pending
read transfer.
A read will normally
be issued at the start of each read period. If the
pending
read transfer has not completed during the read
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 reads, the next read transfer will be issued only after the
pending read 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 can occur.
(Word 12) Timeout for Each Read: Word 12 specifies the time (in hundredths of a second) the Ethernet
Interface will wait for a read transfer to complete before setting the Channel Error bit and Detailed Channel
Status words 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 read period acts as a timeout. (Word 12 should be
zero for no timeout). However, there are two 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 read 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 read period to expire before taking action.