[GreenKeys] Regenerative Loop Driver for Model 15 Teletype

[Mike Douglas]

I  have a new 60ma loop driver for the Model 15 (and related units) up and running that does not require a high voltage supply, nor does it include a switching supply to generate a high voltage. Instead, a regenerative approach is used in which the snubber circuit for the selector magnet coil captures the energy from the mark to space transition and then dumps that energy back into the selector magnet during a space to mark transition. This allows operation of the interface board from a 15v power supply which, in turn, allows it to dissipate very little heat.

The board provides two 1/4" phone jacks for connecting the printer and keyboard cables from the Model 15. Host connections include RS-232 (DB-9 or DB-25 via RJ14 modular connectors) and a 3.5mm audio jack for connecting an AFSK feed (e.g. ITTY). A barrel jack is provided for connection of a 15v AC adapter,

The RS-232 host connection uses ASCII at 9600 baud and includes a handshake line to automatically throttle the sending computer. With a terminal emulator, it's just a matter of turning on the "hardware handshake" checkbox. For vintage computers (e.g., Altair, IMSAI, etc.), virtually all serial boards bring out the CTS pin of the UART to the DB-25 and that will throttle the transmitter without any software changes. This allows the host computer to use ASCII at a standard baud rate instead of having to worry about five bit/45.5 baud support.

Presently, I have ASCII-Baudot translation tables installed for ITA2-TTY and USA-TTY. I've also added the ability to generate ASCII keycodes that can't be generated by the Model 15 keyboard by defining an escape sequence starting with the blank key. The board allows full or half duplex operation and also provides a 5-bit pass-through mode (5 bits within each byte).

The audio interface demodulates AFSK (presently mark 2125 Hz and space 2295 Hz as used with ITTY) and sends the demodulated output directly to the printer.

A configuration menu is entered if ESC is pressed on the host side within 3 seconds after power-on. The menu allows selection of which ASCII/Baudot translation table to use, enabling  a 5-bit pass-through mode, updating the firmware, etc.

I will soon be putting up a video on YouTube that shows a Model 15 in use as the console for an Altair 8800 computer. This provides a good demonstration of the Model 15 as well as the ASCII interface capability of the loop driver.

Mike