[GreenKeys] ASCII/RS-232/AFSK Interface for the Model 15 Teletype

Tue Jun 28 15:21:42 EDT 2022

 Eugene,

The selector magnet is energized/de-energized through T2 and RA5 You'll see the baudot data at RA5.

RC2 through U1 is used to block or dump the charge that has accumulated on C1/C2 into the loop circuit.

RC1 through T1 is used to add additional charge to C1/C2 during a space condition and after current from the selector magnet has decayed to zero.

First, consider the idle marking state: RA5 is high, T2 is on, and 60ma flows from the 15vdc power supply through the selector magnet. At this point, RC2 is high so that the U1 is "off" and C1/C2 are isolated from the power rail.

Now transition from mark to space: RA5 goes low so T2 turns off. The decaying magnetic field of the selector magnet dumps its energy as charge onto C1/C2 through D3. When that current flow reaches zero, D3 blocks the reverse flow of current out of C1/C2, so these capacitors hold their charge. Typical voltage reached on C1/C2 with a pulling magnet selector is 40v-60v on the mark to space transition.

After the selector magnet current reaches zero, the processor runs the boost switching supply formed with T1, L1, D2 and C1/C2 to further boost the voltage on C1/C2 (if enabled).

Next, the processor turns on U1 via RC2 to connect the voltage on C1/C2 to the power rail. D1 blocks the high voltage from reaching the 15vdc supply. At this point, everything is ready for the next space to mark transition.

At the space to mark transition, RA5 goes high to turn on T2. Current flows from C1/C2 through the selector magnet until they have discharged down to the 15vdc supply voltage. Once this point is reached, U1 is turned back off via RC2 and we're back to the idle marking state.

Mike

     On Tuesday, June 28, 2022, 12:42:06 PM CST, W2HX <w2hx at w2hx.com> wrote:  

 Ok Mike. Just a question about the theory of operation of this circuit. From what I can surmise, RC2 on the U2 seems to be doing the signaling for the selector magnets. Meaning, I would expect the baudot code to be present on RC2 (am I correct?) can you elaborate on the functions and interplay of RC1 and RA5? All three seem to be involved in this output circuit.

I think others would also be interested in the theory here. thanks

73 Eugene W2HX
Subscribe to my Youtube Channel: https://www.youtube.com/c/w2hx-channel/videos

-----Original Message-----
From: greenkeys-bounces at mailman.qth.net <greenkeys-bounces at mailman.qth.net> On Behalf Of Mike Douglas via GreenKeys
Sent: Sunday, June 26, 2022 5:44 PM
To: Greenkeys <greenkeys at mailman.qth.net>
Subject: Re: [GreenKeys] ASCII/RS-232/AFSK Interface for the Model 15 Teletype

Eugene,

Unfortunately, no, this board can’t easily be configured to work with an external loop supply.

Mike

> On Jun 26, 2022, at 12:06 PM, W2HX <w2hx at w2hx.com> wrote:
> 
> What a great project! I have a question. Could the circuit be modified to only key an existing loop without the need for this board to generate the loop current itself? I would imagine maybe less components needing installation and some jumpers but I was wondering your opinion of that.
> 
> 
> 73 Eugene W2HX
> Subscribe to my Youtube Channel: 
> https://www.youtube.com/c/w2hx-channel/videos
> 
> -----Original Message-----
> From: greenkeys-bounces at mailman.qth.net 
> <greenkeys-bounces at mailman.qth.net> On Behalf Of Mike Douglas via 
> GreenKeys
> Sent: Sunday, June 26, 2022 12:35 PM
> To: GREENKEYS BULLETIN BOARD <greenkeys at mailman.qth.net>
> Subject: [GreenKeys] ASCII/RS-232/AFSK Interface for the Model 15 
> Teletype
> 
> I recently made a new version of the subject interface board that incorporates the changes I made to the prototype unit that you may have seen in a few videos I posted. I've made a web page to document this new version - see the link below. I still need to write a user manual and finish a small wish-list I've given myself for the firmware.
> 
> https://deramp.com/model15_interface.html
> 
> I have a number of bare PCBs of this new version if anyone wants one - just contact me. I'd still like to work with someone to tweak components and firmware as needed for a holding magnet configuration.
> 
> Mike
> 
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>>>> Jordan Spencer Cunningham's GreenKeys Search Tool:
>>>> https://teletype.net/gksearch 2002-to-present greenkeys archive:
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>>>> 1998-to-2001 greenkeys archive:
>>>> http://mailman.qth.net/archive/greenkeys/greenkeys.html
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>>>> http://comcents.com/tty/greenkeyssearch.html
> 
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>>> Jordan Spencer Cunningham's GreenKeys Search Tool: 
>>> https://teletype.net/gksearch 2002-to-present greenkeys archive: 
>>> http://mailman.qth.net/pipermail/greenkeys/
>>> 1998-to-2001 greenkeys archive: 
>>> http://mailman.qth.net/archive/greenkeys/greenkeys.html
>>> Randy Guttery's 2001-to-2009 GreenKeys Search Tool: 
>>> http://comcents.com/tty/greenkeyssearch.html

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