#: 20208 S1/General Interest
    14-Aug-94  01:55:33
Sb: #20193-RS232 response time?
Fm: Ken Scales 74646,2237
To: Simon R Ashby 100111,2173

Simon -

 > There is a non-real-time setup protocol for positioning the
 > display and crosswires etc, plus a real-time mode where we provide a
 > 'tick' character at 19200 baud every video field sync. If they reply in
 > 3ms with their 10-character data string then we guarantee that the video
 > recording annotation will be deterministic. The data string contains text,
 > numbers, flags and X-Y position for two crosswires.
 > They claim to be unable to respond in 3ms - more like 25ms.

I am, of course, not familiar with the details of what you are doing, so please
consider my comments in this context.

At 19200 baud, assuming 1 start bit and 1 stop bit (i.e., 10-bit characters),
each character will take about 0.52ms to transfer.

Once your system transmits the 'tick' character, it will take 0.52ms before the
other system has received it.  This leaves 2.48ms for that system to respond.

Assuming zero processing time, it will take 5.2ms for the other system to
transmit the 10-character data string to your system -- an absolute minimum
total of 5.72ms after you have transmitted the 'tick' character. So you may
want to review that 3ms specification.

Of course, this does not account for the other 19ms they claim they require. As
Kevin and Bob have said, several factors can influence this number.

Best of luck... / Ken
 --------------------------------------------------------------------------
 Ken Scales   Delphi:KSCALES   Internet:kscales@delphi.com   CIS:74646,2237