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3-19
GFK-1084B Chapter 3 Programming Communications Requests
(Word 8) Channel Number: Word 8 specifies the channel to be used for the write. 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 re-tasked to perform this new
command.
(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 specified in Table 3-1.
(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 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. The pending
transfer can still complete after the period error occurs. 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. 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 write 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.
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.
(Word 13) Local PLC - Memory Type: Words 13-14 specify the location in the local PLC
from where the Ethernet Interface will get the data to be written to the remote PLC.
The size of this area is set by the size of the data written to the remote PLC. Permissible
memory types are given in Table 3-2.
A Channel Command
setup to issue a single
write can have only one
pending write transfer.