[TUHS] Control-T (was top)

Johnny Billquist bqt at update.uu.se
Fri Jun 1 01:02:27 AEST 2018

On 31.05.18 04:00, Pete Turnbull<pete at dunnington.plus.com> wrote:
> On 30/05/2018 23:10, Johnny Billquist wrote:
>> On 30.05.18 02:19, Clem cole wrote:
>>> 1). If you grab ^s/^q from the alphabet it means you can send raw 8
>>> bit data because they are valid characters (yes you can use escape
>>> chars but it gets very bad)
>> But this has nothing to do with 8 bit data.
> [...]
>> What you are talking about is simply the problem when you have inband
>> signalling, and is a totally different problem than 8bit data.
> True, but what I believe Clem meant was "binary data".  He did write
> "raw data".

Clem originally said:
"And the other issue was besides not enough buffering DZ11s and the TU58 
assumes  software (^S/^Q) for the flow control (DH11s with the DM11 
option has hardware (RTS/CTS) to control the I/O rate.  So this of 
course made 8bit data diificult too."

And that is what I objected to. If we're talking about just getting data 
through a communications channel cleanly, then obviously inband 
signalling like XON/XOFF is a problem, but that was not what Clem claimed.

By the way, while I'm on this topic. The TU58 (aka DECtape II - curse 
that name) initially did have overrun problems, and that was because the 
protocol did not have proper handshaking. There is flow control when the 
TU58 sends data to the host, but there is no handshaking, which 
amplifies the problem with flow control on that device. The protocol the 
TU58 uses is called RSP, for Radial Serial Protocol. DEC realized the 
problem, and modified the protocol, which were then called MRSP, or 
Modified Radial Serial Protocol, which addressed the specific problem of 
handshaking when sending data to the host.

But in addition to the lack of handshaking, the TU58 is normally always 
expected to be connected to a DL11, and the DL11 is the truly stupid, 
simple device that only have a 1 character buffer. So that interface can 
easily drop characters on reception. Flow control or not. And the TU58 
is not really to blame. And if you have an updated TU58 which uses MRSP, 
you are better off.

>> RTS/CTS signalling is certainly a part of the standard, but it is not
>> meant for flow control. It is for half duplex and the way to signal when
>> you want to change the direction of communication.
>> It is meant for*half duplex*  communication. Not flow control.
> Actually, for both, explicitly in the standard.  The standard (which I
> have in front of me, RS232C August 1969 reaffirmed June 1981) does
> indeed explain at length how to use it for half duplex turnaround but
> also indicates how to use it for flow control.  It states the use on
> one-way channels and full-duplex channels to stop transmission in a
> paragraph before the paragraph discussing half duplex.

Using RTS/CTS for flow control have eventually made it into the 
standard, but this only happened around 1990 with RS-232E-E. See 
for some background story on that.

And that obviously is way after DEC was making any of these serial 

Wikipedia have a pretty decent explanation of how these signals were 
defined back in the day:
"The RTS and CTS signals were originally defined for use with 
half-duplex (one direction at a time) modems such as the Bell 202. These 
modems disable their transmitters when not required and must transmit a 
synchronization preamble to the receiver when they are re-enabled. The 
DTE asserts RTS to indicate a desire to transmit to the DCE, and in 
response the DCE asserts CTS to grant permission, once synchronization 
with the DCE at the far end is achieved. Such modems are no longer in 
common use. There is no corresponding signal that the DTE could use to 
temporarily halt incoming data from the DCE. Thus RS-232's use of the 
RTS and CTS signals, per the older versions of the standard, is asymmetric."

See Wikipedia itself if you want to get even more signals explained.

If you have a copy of the 1969 standards document, could you share it? I 
was trying to locate a copy now, but failed. I know I've read it in the 
past, but have no recollection on where I had it then, or where I got it 

>> The AT "standard" was never actually a standard. It's only a defacto
>> standard, but is just something Hayes did, as I'm sure you know.
> Except for ITU-T V.250, which admittedly is a Recommendation not a Standard.

This also becomes a question of who you accept as an authority to 
establish standards. One could certainly in one sense say that the Hayes 
AT commands were a standard. But all of that also happened long after 
DEC made these interfaces, and as such are also pretty irrelevant. The 
link to the post in news back in 1990 is from the standards 
representative from Hayes, by the way. So they were certainly working 
with the standards bodies to establish how to do things.


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

More information about the TUHS mailing list