USB-teletype
This is a compact USB to teletype current loop interface. It connects to a computer running Windows, Mac OSX, Linux, Android, or probably anything else, and presents itself as a USB serial port, while the other end connects directly in series with a teletype loop. Data is relayed bidirectionally between the two. Unlike most USB serial adapters, it does not use a dedicated chip like FTDI, CP2102, etc, but instead uses a microprocessor which allows much more flexibility in operation.
- Electrical
The circuit board is physically divided in half. The loop-interface side is 100% completely electrically isolated from the computer interface side. If operating the board without an enclosure, it is very important to insure that nothing conductive comes in contact with it that might bridge exposed contacts and potentially carry high loop voltage anywhere near your computer's USB interface. The loop side is protected against inductive spikes from selector magnets on the line, and should tolerate currents from <20mA to over 100mA. The loop current is sensed non-inductively by a 150 ohm, 2 watt resistor in series with the loop, which provides current to the input side of an opto-isolator. The output side is switched by another optical isolator with an on resistance around 34 ohms. Both input and output can be connected in any polarity. The device has separate connections for input and output, so it can be used in a full-duplex application with two separate loops, but for half duplex operation normally the two connector blocks would be jumpered in series. (they are also completely electrically isolated from each other.)
- Software