[Info-vax] SET TERM /TTSYNC on ancient VMS versions

[Johnny Billquist]

On 2018-08-16 16:09, Chris wrote:
> On 08/16/18 12:35, Johnny Billquist wrote:
> 
>> Well, first of all, using modem controls for flow control is an abuse of
>> the signals. They were never intended for this. And so DEC did,
>> correctly, not use the signals that way.
> 
> Dec terminals were 3 wire, rx, tx data and ground, with xon, xoff
> flow control capability. VT220 had support for that iirc. Some of
> their serial cards had the hardware modem control lines
> available in addition to that. Flow control was needed on early
> systems to prevent buffer overflow etc, but modern systems can often
> run as high as 115k b/s sucessfully with no flow control at all

Not entirely true. DEC terminals always had support for DTR/DSR. But 
that is used to indicate if something is connected or not, and is not 
used for flow control. The DTR/DSR is also included in the MMJ.
All DEC terminals have flow control using XON/XOFF. I'm trying to 
remember if maybe VT05 might be an exception, but I think not. But all 
terminals after the VT05 definitely have XON/XOFF flow control.

Speaking of modem control, DEC terminals actually supported as much of 
the modem control signalling that can be said to have been useful, and 
that was way more than just 3 wires. It's just that people today seem to 
be blissfully unaware of what modem control was meant for.

>> I'm surprised to hear that DEC did have hardware flow control back
>> already in VMS V5. Are you sure about that?
> 
> Using modems, flow control was usually done with the modem control lines
> rts/cts etc, but both xon/xoff and hardware flow control has been used
> all over industry for decades and still is...

No. Modem control, as related to modems, were not about flow control, 
but about handling half duplex communication. And that does indeed go 
back decades, since noone have probably seen half duplex in the last 30 
years. :-)
In rather recent times, the modem control signalling was redefined in an 
RS-232 revision to instead be used for flow control, which had started 
to become the industry standard anyway. But DECs use comes from before 
that, and is based on what the standard actually said at the time.

   Johnny


-- 
Johnny Billquist                  || "I'm on a bus
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