Thicknet/10base5 Test Segment: The Cable is In!

[Tony Duell]

On Fri, Jun 29, 2018 at 9:27 AM, Peter Coghlan via cctalk
<cctalk at classiccmp.org> wrote:
> On 2018-06-28 at 17:05:32 -0700, Chuck Guzis via cctalk wrote:
>
>>
>> The original standard is very old--it dates form 1960--a very different
>> time; DCE was strictly under the control of the telcos, and I suspect
>> that connection to DTE had to be approved by them.  In that sense, pin 1
>> serving as a "protective ground" might have made some sense--and has
>> been grandfathered in.  Note that the DE-9 connector version doesn't
>> include this signal.
>>

I thought the original use of the RS232 interface was between a terminal and
a modem sitting right next to it. In which case there is not likely to be much
voltage between the protective grounds of the 2 devices (well, unless there are
serious problems with your mains wiring!). Longer distance serial interfaces
were originally current loop which were electrically isolated at one
end, originally
using high-speed relays and later opto-isolators.

>
> A telephony connection is the most plausable theory I have come across yet.
> I can remember devices that looked like large junction boxes with a ground
> connection that were installed where an overhead telephone line entered a
> building.  They contained a fuse in series with each line conductor and
> a surge arrestor consisting of a spark gap and/or a VDR from each conductor

The one I've come across is called a 'Protector, Heat Coil & Fuse'. It has a
cartrdge fuse and a heat coil (basically a resisance coil with a spring-loaded
soldered joint that is designed to open if the thing gets hot enough to melt the
solder) in sereis with each line wire and a spark gap between each wire and
ground. I think they were more trouble than they were worth, the one I have
was officially modified by removing the spark gaps and replacing the (cartridge)
fuses with solid metal bars, leaving only the heat coils in-circuit.

> to ground.  I think the theory was that they might provide some protection
> against brief high voltage spikes induced onto the line by thunderstorm
> activity.  I think they might have been more trouble than they were worth.
>
> Although I have never come across one, if such a surge protection device was
> available for a -232 circuit, I could see pin 1 of the DTE or DCE (whichever
> end the device is at) being a semi-plausable place to pick up the ground
> connection for it.  However, I can't see any reason for continuing the
> protective ground connection any further on beyond this device.

There was a thing called a 'barrier box'. At one time in England you had to
rent the modem from the Post Office (who had a monopoly on telephone
systems [1]). You also had to have a 'barrier box' between your RS232
device and the modem. This consisted of a low-rating fuse (50mA?) in series
with each RS232 wire (apart from pin 1, Protective Ground) and a pair of
zener diodes (25V?) in inverse-series between each RS232 wire on the modem
side of the fuse and Protective Ground. So 24 fuses and 48 zeners in
all. Protective
Ground was passed through it AFAIK.

[1] I seem to remember that Hull had its own telephone company, but I suspct the
same rules applied.

Some early serial interfaces had internal fuses/zeners. The HP11206 (modem
interface for the HP9830) does, although the HP11205 (RS232 interface for
serial printers, etc) and HP11284 (async/sync serial interface) do not. In the
case of the HP11205, I suspect it is because it was never intended to be used
with a modem. in the case of the HP11284, I suspect the requirements had
eased by the time it was introduced.

It appears the British Post Office were very worried about excessive voltages
on their lines. I have an HP telephone line analyser and the UK version (which
is the one I have) has an extra PCB of zeners and fuses inside (it's desribed
in the service manual as the 'A44 Protection Board Assembly' and it states
that the 'A44 Assembly is only fitted to instruments intended for use in the UK'


For some unknown reason, you were supposed to use a barrier box on the
GPIB interface of the PERQ if you linked certain printers to it (like
the Versatec
electrostatic thing). The unit was just an RS232 breakoutbox with a couple of
specially-wired cables to get the signal lines on protected pins and the
protective ground line on the GPIB sheld pin.

-tony