On Fri, Jun 29, 2018 at 9:27 AM, Peter Coghlan via cctalk <cctalk@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
