vt220 question

John> On Tue, 4 May 2004, Jay West wrote: John> Well, two things. First, a 'real electrical "BREAK" signal' is John> not a signal per se, but a momentary interruption in the actual John> signal current flowing in the terminal circuit. This makes John> sense in a 'current loop' environment, such as DC-loop teletype John> circuits. ... John> I am not sure that the newer 20ma current loop devices - John> ASR33s, and DEC (and other) devices that use 20ma loop serial - John> will use this actual electrical 'break' for an "ATTENTION" John> signal. Anyone...? Yes, they do. In the ASR33, Break functions exactly as you described it. On newer devices, Break is generally defined as "open the circuit for several character times" or "open the circuit for at least 200 ms", something like that. John> On my VT220, as connected to a VAXStation 3100, depressing the John> 'BREAK' key on the LK401 causes an immediate processor halt and John> the display of a monitor prompt... >>> . Since I have it John> hooked up using the RS232 port, and since there is no such John> thing as a real interrupting 'break' in RS232 - it must be some John> other code... or, is it? Break most definitely IS defined on RS232. There's a one to one correspondence between current loop and RS232 levels: closed loop is mark and open loop is space, and mark and space correspond to the two RS232 signal levels. (I may have mark and space reversed... the McNamara book has all this but it's buried.) So Break defined as "send space for > 200 ms" works perfectly well in RS232. The UARTs at the computer end either recognize break directly and report it explicitly, or (in older ones) the driver uses a definition of the form "if I get a framing error associated with a character code 0xff, it's a break". On DEC consoles, Break has traditionally been the "talk to the console microcode" attention signal. On other terminal lines, Break if often translated to control/C (it is on RSTS, for example, or at least it can be configured that way). Every DEC terminal I know of has support for Break for this reason. paul