Re: Serial breaks
der Mouse <mouse <at> Rodents.Montreal.QC.CA>
2006-04-07 16:20:11 GMT
> The problem is what befalls the VAX whenever the PC on the other end
> of the cable is rebooted (or powered on). As part of its
> initialisation procedures, the PC BIOS appears to send "break"
> characters/sequences (what exactly is it?) down the serial cable,
A break condition is not a character. (It also may not be what's
happening here - see below.)
To understand what a break condition actually is, you need to
understand how characters are represented on a serial line. You may
already know some of this; to avoid back-and-forth explanations, I'll
assume you know just about nothing.
Serial lines such as your VAX's console are, electrically, RS-232.
(They often do not follow the other parts of RS-232; for example, some
(all?) VAX consoles use a DE9 connector instead of a DB25.) RS-232 (an
EIA standard, EIA RS-232-C I think it is in full) is a voltage-based
spec, specifying two states for data transmission: two different
voltages on the data line (referenced to the signal ground line).
There are a bunch of other lines as well, carrying non-data information
such as "ring indicator" - the standard was designed for use with
modems and the like. (These voltages are something like, +3 to +15
volts is one state and -3 to -15 volts is the other. ±12V is perhaps
most commonly used.) You may sometimes see these two states called
"mark" and "space", largely for historical reasons.
When idling (ie, no data being sent), the line is in one of these two
voltage states. Since it is the one corresponding to a 1 data bit, it
is usually taken as "high" when drawing graphs. When a character
starts, the line is driven to the other state ("0") for a particular