[GreenKeys] Regenerative Loop Driver for Model 15 Teletype

Mike Douglas deramp5113 at yahoo.com
Wed Dec 8 16:17:25 EST 2021 

 
Harold,

1) The rise and fall times of the capacitor charge and discharge are virtually identical - just under 2ms. When the capacitor voltage peaks after turning off the selector magnet, the inductor current has reached zero.

2 & 3) The drive of U1 and T1 are opposite polarity, so I have to use separate pins, but more importantly, the switching time of the opto MOSFET is very slow compared to the directly driven MOSFET. Therefore, I switch the charge/discharge direction of the opto MOSFET half way through the bit time. That way the opto is fully turned on or off by the time the selector magnet changes state. This results in the capacitor voltage waveform looking basically the same as the data but shifted by about 11ms. The rising and falling edges of the capacitor waveform are the 2ms or so it takes to charge and discharge the capacitor. The voltage reached by capacitor waveform is 60v-80v in the optimal configuration. Lower capacitance gives higher voltage, but results in slightly less current at the end of discharge. Too much capacitance and the voltage gets too low and again results in slightly less current build-up at the end of discharge.

Mike

     On Wednesday, December 8, 2021, 02:36:06 PM CST, Harold Hallikainen <harold at w6iwi.org> wrote:  
 
 Very clever! A few comments:

1. Does the charging of C1 by the flyback voltage significantly increase
the fall time of the current through the selector magnet?

2. What does the waveform at the top of C1 look like?

3. Since it appears that U1 and T1 should switch on at the same time, I
wonder if both could be driven by RA5. RA5 through resistor through U1 LED
to ground, and RA5 to gate of T1. It would not save any parts, but would
save a pin. Perhaps the increased drive to the FET by using a separate pin
makes the existing circuit better.

In any case, very clever! Thanks for sharing.

Harold




On Wed, December 8, 2021 7:57 am, Mike Douglas wrote:
>  You can see what I presently have running here:
> https://deramp.com/downloads/teletype/Model%2015/loop%20interface%20board/
>
> Mike
>
>      On Tuesday, December 7, 2021, 09:41:41 PM CST, Harold Hallikainen
> <harold at w6iwi.org> wrote:
>
>  Very nice! It would be  interesting to see a schematic of the loop
> keyer.
>
> Harold
>
>
> 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
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>
>
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