Orphaned Response

[rjn]

re:  *positive* flow control ("I'm OK for another 347 characters now").

> ETX/ACK comes close to being standard.  The sender embeds ETX controls in the
> output  stream; the receiver  returns ACK controls each time it's freed up 80
> bytes in its input  buffer (at least; this is the part that's not  standard).

HP terminals use a similar  protocol:  ENQ/ACK (although most HP systems seem
to be moving to  XON/XOFF,  also  supported  by  current HP  terminals).  The
difference  between ETX/ACK and ENQ/ACK is that the ENQ is sent BEFORE the 80
characters  of data,  instead of  afterwards.  Of course, once the stream has
started, there's really no difference.

An interesting  question about these protocols is:  "How long do you wait for
the ACK (or  XON),  and  what do you do if you  don't  get  it?"  HP  systems
usually wait 5 or 10 seconds for the ACK, and then transmit anyway(!).  (This
makes it very easy to recognize when the terminal has not been configured for
ENQ/ACK - you get a burst of characters every 5 sec.)

Our  XON/OFF  hosts  seem  to  wait  forever  for  the  XON, so our  XON/XOFF
peripherals,  such as the LaserJet, are adopting a strategy of sending an XON
every 5 seconds  or so when no data has been  received  for 5 sec.  This gets
the line going again if an XON has been lost for some reason.

Regards,                                                Hewlett-Packard
Bob Niland                                              3404 East Harmony Road
hplabs!hpfcla!rjn                                       Fort Collins CO  80525