- On-board conversion of ASCII text from the computer to USTTY/ITA2 (often incorrectly referred to as Baudot-Murray) on the current loop and vice versa (for 5-bit machines)
- This can be disabled if your software passes USTTY/ITA2 encoded text to the serial interface; if disabled, the board does no translation of encoding
- Pass-through of ASCII encoding for 7/8-bit machines
- Other character encodings – encodings – customizable translation tables can be configured to enable other encodings outside of ITA2 USTTY and ASCII
- 45 and 110 baud rates have been tested the most, but other common rates (and probably all uncommon rates for that matter) used in teletypery should work without problems
- 20ma and 60ma current loops supported (current levels both lower and higher are safe depending on the exact optoisolators used, though it would be unusual to see other currents in use)
- All typical loop voltages supported-- in theory voltages as low as ~5VDC or as high as ~200VDC are possible depending on the optoisolator used in your particular iteration of the board
- Will this board work with model X teleprinter? If it uses a current loop, then most likely. Pretty much any ITA2 or ASCII, 45+ baud, 20ma or 60ma current loop machine should work with this board. Even if you have an unusual type of loop, baud rate, or 5-bit encoding, chances are the board can still be made to work with your oddball machine.
- Will this board work with X computer or X operating system? Almost certainly-- it shows up as a simple serial interface as far as the computer's concerned, so almost anything remotely modern should be able to drive it. It has been most extensively tested on a few flavors of Linux, MacOS, and Windows, however. The computer must also support USB, of course.
- Does this board demodulate RTTY? No, this board is not a TU. However, it could be used in tandem with demod software such as fldigi to act like a poor man's TU, as has been done with ttyBash. The designer of this board has also designed an RTTY demod board. I have considered doing a run of these boards one day, too.
- Does this board require an external loop power supply? Yes. This board does not provide any power to the loop.
- Does the polarity of the loop matter? Nope. There is a bridge rectifier added to the RX_LOOP circuit, which allows for the loop to be wired in either polarity.
- Does this support full duplex or half-duplex? Yes. <This should probably move to be its own section down below>
- The most common mode is probably half-duplex, meaning you have a single current loop to which your teletype keyboard and printer are wired in series (or just the printer if it's an RO unit). This is achieved by bridging the TX and RX terminals with a bit of hookup wire between the center terminals <image to come to better illustrate this>. Then the single loop must be wired to the outermost terminals.
- Full duplex involves two electrically isolated current loops-- one for your keyboard (RX TX on the board), and one for your printer (TX RX on the board). Full-duplex can be achieved by removing the jumper between the TX and RX terminals and simply wiring the two loops to the two pairs of terminals <image to come to better illustrate this>.
- Are there any recommended loop power supplies? There is a great selection of tried and true supplies outlined here <link to be added later>.
- Must I use a fuse on the current loop circuit? Technically no-- it's not very likely you'll ever need it (assuming you correctly current-limit your circuit when you first build it) since if a dangerous over-current occurs on the loop, the current limiting resistor you put into series will almost certainly fail open, protecting the board and the teletype's selector magnets. However, I prefer to be extra safe and fuse my loops to protect my boards and magnets from the unlikely event of an overcurrent that causes the resistor to fail closed or the resistor only fails after the board and/or magnets are fried. I also prefer to fuse to protect from the more likely event that I do something stupid.
One of the great things about this board is that all of its components have through-hole counterparts, so a somewhat solder-savvy person can procure parts and assemble boards by hand. Those with the skills and equipment for SMT can, of course, stick with the SMD components, or a run could be manufactured at any reputable fab house.
Schematics, BOM, &c.
more to comeAvailable for the version of the board I made in 2019 here. Note that since that time it is likely some components may have been phased out, but this is common. In the past a newer, better compatible component has always been made available in lieu of the old one.
Tips for Assembling Through-Hole