---------------------------------------------------------------------- Message-ID: <61e5c682-f415-4d1a-9258-7f4763d16f4d@googlegroups.com> Date: Wed, 28 Dec 2016 20:43:01 -0800 (PST) From: HAncock4 <withheld@invalid.telecom-digest.org> Subject: Re: Telex and TWX History On Tuesday, December 27, 2016 at 10:40:35 PM UTC-5, Bill Horne wrote: > In order to tell 60-speed and 100-speed machines apart, any call form > the "regular" DDD network to a 100-speed machine (the ones with the > newer x10 area codes) was automatically converted from Baudot to ASCII > code, and the speed adjusted. That was the source of my problem: I was > using a "100 speed" machine to call from a "60 speed" phone number, so > the WADS office was trying to convert Baudot to ASCII, etc. Would anyone know how the speed was adjusted? I would guess that in a long message the 100-speed machines would overrun the 60-speed machines unless there was some sort of intermediate storage buffer? ------------------------------ Message-ID: <20161231005135.GA26730@telecom.csail.mit.edu> Date: Fri, 30 Dec 2016 19:51:35 -0500 From: Bill Horne <bill@horneQRM.net> Subject: Re: Telex and TWX History On Wed, Dec 28, 2016 at 08:43:01PM -0800, hancock4@bbs.cpcn.com wrote: > On Tuesday, December 27, 2016 at 10:40:35 PM UTC-5, Bill Horne wrote: > > > In order to tell 60-speed and 100-speed machines apart, any call form > > the "regular" DDD network to a 100-speed machine (the ones with the > > newer x10 area codes) was automatically converted from Baudot to ASCII > > code, and the speed adjusted. That was the source of my problem: I was > > using a "100 speed" machine to call from a "60 speed" phone number, so > > the WADS office was trying to convert Baudot to ASCII, etc. > > Would anyone know how the speed was adjusted? I would guess that > in a long message the 100-speed machines would overrun the 60-speed > machines unless there was some sort of intermediate storage buffer? I'm glad you asked: I spent many hours figuring out exactly how this was accomplished, and I'll pass this info along. We went over this in the Digest four or five years ago, but I figure the old stories are the best. ;-) TWX 100-Speed machines had a "Restrain" feature, which was a circuit that would pause the tape reader (and IIRC, also locked the keyboard) when the 100-speed machine was sending to a 60-speed machine and the message was too long for the buffer at the WADS office, which is where ASCII was converted to Baudot, and 110 baud was converted to ~45 baud. The operator at the 100-speed machine would see the "Restrain" light come on, and that was their queue to wait until the 60-speed machine at the other end had printed enough characters so that the buffer in the WADS office could accept more from the 100-speed machine. Of course, if the 100-speed machine was sending from its tape reader, it would stop until the Restrain light went out. I was curious how this feature worked, and I arranged for a keyboard-to-keyboard call between our Model 35 and another machine in our building, both of which were using regular dial tone and were not part of the TWX network. When the operator of the other machine sent a "X-OFF" command, which stopped the reader at my machine until he sent a "X-ON" command, the "Restrain" light did not come on. In other words, his "X-On/Off" keys would properly control my tape reader, but did not cause my machine to go into "Restrain" mode. This seemed like a small matter, but I wanted to know, and so I drew up a list of all the possible signals another machine or the WADS speed/code converter could send. If the "DC1" and "DC3" ASCII codes weren't being used to control the tape reader on the remote machine, then I figured that there had to be some different signal which was unique to TWX. In one of my rare occasions of looking for the simplest solution, I dialed my TWX machine from the testboard, and fed it the "Mark" modem tone from the test oscillator. The TWX sat quietly and did not drop the call, for as long as the tone was on the line. If I interrupted the tone for more than a fraction of a second, the TWX machine dropped the call and turned itself off after a few seconds, so it was clear that dropping the carrier signal wasn't part of the "Restrain" mechanism. I had a moment of inspiration, and added another oscillator to the circuit after I established a call to my Model 35 machine: one was already set to the "Mark" tone, of course, and the other I set to the "Space" tone. Sure enough, the "Restrain" light came on when both Mark and Space tones were present at the same time. I think Bell Labs chose to use that dual-tone signal for that purpose because there was no ASCII code to signal "Lock the other machine's keyboard". Anyone with first hand knowledge is invited to tell us the rest of the story. Bill -- Bill Horne (Remove QRM from my email to write to me directly) ------------------------------ ********************************************* End of telecom Digest Sat, 31 Dec 2016