[GreenKeys] Computer to teletype module...

[John Nagle]

> From: Jordan Spencer Cunningham<js at cunni.co>
> Subject: Re: [GreenKeys] Computer to teletype module...
> Message-ID:
> 
> There are two major recent USB<-->TTY designs of which I am aware. There
> are likely more, but these are the two that are the best documented thus
> far. Neither is superior than the other; both have their uses and were
> designed with different approaches. Their creators are on this list and can
> speak more to this if they wish. Consider this a customer review of sorts.
> 
> The first I came across and the one I ended up building was designed by a
> fellow who shares your name, Eric Volpe. The interface is documented here:
> http://heepy.net/index.php/USB-teletype  While there was a run of these
> professionally made on a size-efficient PCB with SMD components around a
> couple of years ago I understand, that supply was depleted by the time I
> started asking around, so I ended up hand-assembling mine with through-hole
> equivalent parts and an Atmel32u2 dev board. 

    That's a nice little design. Does Baudot-ASCII translation, too.
Requires an external loop supply.

> The second I came across is also a great option, but I'm not nearly as
> familiar with it. It is John Nagle's recent design, documented here:
> https://github.com/John-Nagle/ttyloopdriver. It's similar in that it's also
> a USB/TTY interface, but its "current loop", if you will, is
> self-contained, meaning you can connect your machine right up to it without
> needing a current loop power supply. It's all powered from USB using a
> clever bit of engineering. From what I read on the GitHub readme for this
> design, its firmware does not perform ASCII translation (someone-- John or
> Steve-- correct me if this is wrong), but there is, of course, software
> that already exists like Heavy Metal that makes this easier to deal with.
> John also wrote baudotrss<https://github.com/John-Nagle/baudotrss>, which
> looks like it parses RSS feeds as well as sends/receives SMS. While you
> could probably find through-hole equivalents for the components used in
> this design, it's been designed strictly with SMD components. SMT is beyond
> my skill, nor do I wish to invest in the hardware that's really needed to
> do it right. It's definitely a good thing to learn if you want to truly get
> into modern electronics, though. This was the one main factor in why I
> decided not to use this design.
> 
> Either of the above designs could be professionally manufactured if hand
> soldering and figuring out through-hole equivalent components is not your
> thing. The problem I've found with this is that it requires a relatively
> large run to make the price per unit palatable, and I don't know that there
> is enough interest to have a large enough run done.

    Exactly, My design could easily be made by a PCB assembly shop,
but it's not an easy hobbyist project.

    There are no through-hole equivalents for many of the parts
in that design.  It has a specialized switching power supply which
uses modern switching power supply components. Those just aren't made
in through-hole. The layout around the switcher has to have very short
lead lengths or it won't work at all.  Having been forced to SMD,
I went as far as I could towards all SMD.  Once you're set up to
assemble SMD, it's easier to place SMD parts than through-hole parts.

    If there was a market for 100 of those things, it might be
worth having them manufactured.  Less than that, not worth it.

    I have an old, easy to build through-hole design at

http://www.aetherltd.com/connecting.html

which requires an external 120VDC power supply and an external
USB to serial converter.  I built and used several of those.
But surplus 120VDC power supplies are now hard to find.

				John Nagle