On 2016-01-01 23:08, Mark Pizzolato wrote:
On Friday, January 1, 2016 at 1:02 PM, Johnny Billquist wrote:
Um. What does 7B and 7P stand for here? They are not "normal"
designation for 7 bits with some parity... The letters I know of are N - None, S
- Space, M - Mark, O - Odd, E - Even.
And if you are using anything that manage to print something visible when
outputting a NUL, that sounds like something broken somewhere else.
Sounds like the current simh code is trying to cover up when people have an
error somewhere else in their setup?
Actually, no. Some simulated systems have very particular expectations of the
terminal devices which are connected to the system. They'll output 7bits of
data with a parity bit. In general the parity bit isn't useful for essentially
any
modern system running the simulator so 7P and 7B allow the parity bit to be
stripped off before the data is delivered to the local session. Besides parity
bits, as Dave Bryan points out some of these simulated systems presume
they're talking to ASR Model 33 teletype terminals. When talking to these
slow mechanical devices the simulated system often will insert NUL or
RUBOUT or other characters into the output character stream as non-printing
delays to accommodate the slow mechanical behavior of the terminal
when it may be repositioning the print head. If you happen to be running
a simulator with a real Model 33 teletype terminal then you would want to
run the console port in 8B mode so all of the generated characters and
parity bits get to the terminal, otherwise most folks will want a filtered mode.
I would like to point out that this is in no way ASR-33 specific. Using
NUL fillers for delays was needed on *lots* of terminals, including even
a VT100 when running at higher speeds and doing some operations (like a
linefeed).
So NUL characters was used a lot of the time. And there are also lots of
other control characters defined in ASCII. And programs sometimes use
them. And they have defined behaviors. And simh deciding to filter them
all out is basically wrong. If you have a specific environment, in which
you are not actually using a proper ASCII conformant terminal, then
fine, have some way for simh to accommodate such brokenness. But don't
assume it, and don't enforce it, and don't break everyone else because
of it.
Some/most MUX devices (DZ, DC, DL, TTO) have translation mode options to
configure this detail:
8B means pass 8 bit characters unfiltered.
7B means just strip parity bits.
7P means strip parity bits and only allow control characters which are in
the PCHAR mask to be output and also ignore output of RUBOUT characters.
UC means do 7P and convert lower case to upper case
Just learned about those designations. Thanks.
Johnny
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: [email protected] || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
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