[GreenKeys] Use of Volpe Board

[epvgk at limpoc.com]

For people who are considering using this board to interface a TTY loop with a computer, and
who don't already have an existing current loop, here is what I'm using. 

This power brick is kind of overkill but not too expensive, and more importantly to me at least
is very RF-quiet, so if you do radio stuff too it doesn't produce much RFI at all. 

https://www.aliexpress.com/item/33039935845.html

You also need to limit loop current to (ideally) 60 mA for 14/15/28/etc type machines, so put
it in series with a 20+ watt 2K resistor like 
https://www.parts-express.com/2k-ohm-20w-resistor-wire-wound--017-2k
(it will get hot, give it a heat sink or space to breathe!)

For 20mA machines you could use the same resistor but a 48VDC supply and probably be ok, if 
the teletype doesn't already provide loop current. 

eric


On Fri, Sep 06, 2019 at 06:15:02PM -0700, epvgk at limpoc.com wrote:
> On Fri, Sep 06, 2019 at 04:09:44PM -0700, Ken Wiebe wrote:
> > I asked Jordan this question but he wasn't sure. I asked it in a slightly different way, but here goes:
> > 
> > Would it be fair to compare the Volpe board to many of the other current loop-to-serial adapters, except that the Volpe board uses USB instead of RS-232?
> 
> The goal is the same, to connect a teletype loop to a computer, so the computer can send
> and receive text to/from the loop. 
> 
> The thing is that few computers and no laptops from the past 10+ years have RS-232 serial 
> ports - USB is used for everything.  This board was an attempt to make it possible to keep 
> connecting teletype loops to computers without having to constantly fiddle around trying to 
> keep an old computer and operating system functional just for the RS232 port. 
> 
> There are also USB-RS232 adapters, but a) most of them can't be made to speak 45 baud, b)
> they still require a separate current loop supply, and c) they cost more than this board
> anyway. This one is also designed specifically for teletypes. 
> 
> > I have a little experience hooking up a 33 and a 35 to computers using the more traditional RS-232 serial ports on the computer side.
> > 
> > I realize the Volpe board has added functionality (which is one reason it interests me). But since the TTY side of the Volpe board looks (to me) just like the TTY side of the current loop/RS-232 adapters, my assumption is that it can be used in the same way.
> 
> Yes, it definitely can be used that way - I designed it mostly for older 45 baud baudot 
> machines that you'd like to talk to in ASCII, since that's what I have, but you can tell 
> it to not translate anything at all and just act like a pass-through too.  It also doesn't 
> require you to have a physical RS-232 serial port. 
> 
> > What is prompting this question is that the Volpe board doesn't seem to be "marketed" as this type of device, and also the stated limitation that it does not provide its own loop current.
> 
> It doesn't provide loop current itself because it's meant to "look" just like a 14/15/28/etc
> which expect to be in series with an existing loop as well as other machines. My loop consists
> of a 14TD, a 14 reperf,  a model 19, and a model 28, all in series with a big fat 2K resistor
> and a 150VDC power brick.  
> 
> The one-computer <---> one teletype application is sort of a special case where it's still
> a loop (or two, if you're full duplex) but still needs a "loop" supply. 
> 
> > 
> > So here's the crux of my question: Let's say I hooked up the Volpe board directly to a model 33 - one pair to TTY send and the other pair to TTY receive. Would all the pieces be there to tx/rx to a USB-connected computer?
> 
> I think so - I don't have a 33, but from what people have told me, they provide their own 
> loop current, so just connecting the appropriate wires to this board would make your 33 look 
> like a full duplex 110 baud USB serial terminal.  
> 
> But it's unclear to me if f33's can be made to provide power for both send and receive loops -
> if not I guess you'd still need a loop supply for one side?  I hope someone else here with
> 33's can clarify this. 
> 
> > 
> > If not, I guess that means that the current loop/RS-232 circuits I've been using thus far somehow inject current into the TTY side (?)
> > 
> Either the adapter circuit or the 33 itself would have to, yes. 
> 
> > What would I need to add to make the Volpe board work in this scenario?
> 
> just something like a 48VDC power supply and a 2W resistor. 
> 
> eric
> 
> 
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