Tty Loop

Marc i. Leaveyt M.D. WA3AJR 6 Jenny Lane Pikesvifte MD 21208

I don't do this often, but in the spirit of ihis month's holiday, I hope you'll bear with me.

In the beginning, the Teletype Corp. created the Model 12, and it was good. Next came the Model 14, and ii was good. Then came the Model 15, and it, also, was good. Soon thereafter came the Model 19, and it was very good. And then came the Model 28, and rt was very good, indeed! But then came the Model 33r and it was the best yet. Ouch!

Well, you get the idea. Here I am with notes from many of you about the Teletype Model 33. Major Gene Pfeiffer of New York asks tremendous variation in what FSK signals you'li encounter when tuning across the HF spectrum. Don't be discouraged—hopefully some of the material presented this month will hefp.

Andrew Janer WA1VRZ in East Haven CT is an ambitious soul who hopes to hook up a Model 33 to a Timex computer We aim to please, sir.

The Teletype Model 33 teleprinter is either an electromechanical marvel or a nightmare, depending on how you iook at it. Using a typing cylinder, kind of a "squooshed" Selectric bail, it chugged along at 110 baud—equivalent to 100 words per minute. ASCI! code, uppercase only with a "Teletype standard" 72-character line length. Despite these limitations. many oMhese monsters found their way into amateur radio stations, interfacing the Model 33 is made difficult by the fact that there is no single standard version of the 33, but a variety of options contained in a panel on the right side of the machine, the Call Control Unit (CCU). Various CCUs were manufactured {ihe Model 33 is not out of production) for line service and telephone systems, as wefl as for computer interfacing. Jf you can get it, the I/O 33 is the easiest of all to interface. Th\s CCU is identified by six push switches along the front of the CCU labeled MOTOR OFF, REMOTE, REQUEST, EOM, LOCAL, and PUNCH. Usually, that is _ you see, there are different ones. too. And you can't always tell a CCU by its buttons.

Anyway, if you have one of those, you'll find a terminal strip inside, at the rear of the CCU1, On mine, it was mounted vertically under the bank of Molex* connectors. On yours, who knows? if you have one like mine. connect the brown/yeJlow and white/blue wires to terminal 5, and the violet wire to terminal 9t and move the blue wire from the 750-Ghm tap to the 1450-0hm tap on the power resistor behind the power supply inside the teleprinter, Then connect the interface through the connections shown in Fig. 1h after first checking for unwanted voltages and the like.

If you re a trifle less than brave, as I am (not), you might be interested in the circuit of Fig. 2. This is a scheme to use an optoisolator. such as a 4N35h to isolate the computer or interface from the deadly voltages that may be lurking around the Model 33. i highly recommend such isolation.

Let's say that you have one of those Model 33s that isn't equipped with an I/O CCU., now what? Weil, you stitl should have the terminal strip, often labeled 151411, at the rear. If you lack that, trace out the wires coming from the selector magnets. They should enter a printed circuit board, the Selector Magnet Driver board. Pin 6 from this board should be ground; pin 11 is the signal input.

The wire going to pin 11 p by the way, should be coming from the transmitter distributor. That means the keyboard signal should be available there. Again, if you have a custom-wired 33, all bets are off. but you should be able to trace out the wiring and go from there

Assuming you have been able to identify the proper teads to connect to, and have interfaced the 20 mA TTY loop with the more


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