cation decides to cancel whatever part of the "BBBB" has yet to leave the serial port, and sends a
"transmit flush" command to the adapter. If the adapter had been lying about being done sending
"AAAA," it might still not be done, and the flush could purge the transmit FIFO of some of the A's
(which is not what the application was expecting) in addition to the B's.
Timing:
In some applications, the receipt of the "I'm done" indication is used as a timing reference point. For
example, an application could send a data stream like this:
COMMAND................................................................
The extra periods following COMMAND would be just used to establish a timing interval: the applica-
tion would know that once the serial port said "I'm done," the receiver would have had at least 64
character times to process COMMAND. If the "I'm done" indication was sent early, the application
might then do its subsequent action too early.
There are many other communication protocols in which time periods are counted from the point
that a message has been delivered. Any time you change the timing of the "I'm done" indication, you
run the risk of interfering with such protocols.
What Should I Use?
There are two ways you can configure Tx Ack Advance:
Standard:
If you're using a baud rate of 57600 or less, you probably don't have a problem. The 1 millisecond
delay will not significantly affect your throughput, and you can select this option. This option is also
a good choice if you are running an application in which throughput is not an issue, or one in which
guaranteeing 100% correct operation is important enough that you'd prefer not to risk having prob-
lems.
Faster:
If you're using a baud rate above 57600, you can simply try increasing this setting, and see whether it
improves performance (in which case, great!), causes problems (in which case, don't use it), or
doesn't make any difference (in which case it doesn't matter what you do).
USA-19HS - User Manua
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