On Tue, December 7, 2021 6:51 pm, Mike Douglas via GreenKeys wrote:
> 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