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TCP/IP Ethernet Communications User’s Manual - August 1997 GFK-1084B
(Word 8) Channel Number: Word 8 specifies the channel to be used for the read. This value
must be in the range of 1 to 16. If the channel is out of range, a command error indication
will be placed in the COMMREQ Status word. If the channel number is the same as a
channel already in use, the channel will be retasked to perform this new command.
(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 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 specified in Table 3-1.
Table 3-1. Time Unit Values for Read/Write Repetition Period
Value Meaning
1 hundredths of seconds (10 ms)
2 tenths of seconds (100 ms)
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 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. The pending
transfer can still complete after the period error occurs. 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. The transfer can still complete even after a timeout occurs. 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 need not be specified because the read period, in effect,
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 timeout errors will be
reported by the Channel Error bit.
A Channel Command
set up to issue a single
read can have only one
pending read transfer.