[Simh] SIMH console settings for escape sequence support

[Mark Pizzolato]

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.

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

Some simulators have an additional mode KSR which does UC and drops all characters >= 0140 (`).

- Mark


> On 2016-01-01 21:53, J. David Bryan wrote:
> > On Thursday, December 31, 2015 at 16:19, Mark Pizzolato wrote:
> >
> >> The PCHAR mask is actually only used when the terminal device
> >> (console) is in 7B or UC mode.
> >
> > That should read "7P" mode, not "7B" mode.
> >
> >
> >> In the interest of fewer problems in the future for various folks,
> >> the default PCHAR mask has now been changed to include ESC and ENQ in
> >> addition to the previous BEL, BS, TAB, LF and CR.
> >
> > The intent of 7P mode is to mask off non-printing characters --
> > basically, those not interpreted by a Teletype.  By contrast, 7B mode
> > allows all 7-bit characters through.
> >
> > Some simulated systems output control characters, e.g., NULs as pads,
> > which wind up printing junk characters on the screen in 7B mode.
> > Also, some simulated programs rely on control characters not printing
> > when editing command lines for example.  These rely on mode 7P to work
> properly.
> >
> > Rather than changing the non-printing character mask to allow
> > non-printing characters to print, it would make more sense to change
> > your device default from 7P to 7B.
> >
> >
> > On Friday, January 1, 2016 at 15:03, Paul Koning wrote:
> >
> >> More... it occurs to me that many operating systems and/or
> >> applications do their own filtering of control characters, so in fact
> >> a good setting for the pchar mask is all ones.  In other words,
> >> 37777777777 octal or ffffffff hex depending on the simulated machine.
> >>
> >> I wonder if that should be the default, instead of 23600.
> >
> > That would make modes 7P and 7B identical.  Again, changing the device
> > default would be better.
> >
> >                                        -- Dave
> >
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> 
> 
> --
> Johnny Billquist                  || "I'm on a bus
>                                    ||  on a psychedelic trip
> email: bqt at softjar.se             ||  Reading murder books
> pdp is alive!                     ||  tryin' to stay hip" - B. Idol
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