vt220 question - test-drb@ccmp.vtda.org

4 May 2004

...
 On my VT220, as connected to a VAXStation 3100,
depressing the 'BREAK'
key on the LK401 causes an immediate processor halt and the display of a
monitor prompt...   >>>  .   Since I have it hooked up using the RS232
port, and since there is no such thing as a real interrupting 'break' in
RS232 - it must be some other code... or, is it?  The VT220 Programmer
Pocket Guide just says that he first 5 top-row keys are "local function
keys and do not generate codes."

  But it still doesn't tell me what the hardware does when pressing
'BREAK'.   IF no one knows, I'll have to get out the break-out box and
see if I can trap it. 

BREAK on typical RS-232 terminal simply sets the line to the SPACE condition
for a number of character intervals. This causes a framing error at the
receiver (no stop bit) and many system will time this and if it persists
long enough will interpret it as a system attention function.

Better terminals will generate a timed pulse when break is depressed, some
terminals simply send a space condition for as long as the key is held down.

Regards,
Dave Dunfield

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