[GreenKeys] RS232 to TTY Loop Boards

[John Nagle]

    As the designer of that circuit, I should make a few remarks.

    First, you still need a 120VDC supply with this.  It doesn't
generate the loop current.  I did it this way because I'd
obtained several 120VDC regulated open-frame supplies at
a surplus store.  A more modern design would generate
loop current from a low voltage, probably from a USB port.

    Second, this design assumes the input serial ports
go both negative and positive.  Some modern serial ports
are 0 to +5.  That won't work.  The board will work with
classsic desktop PC serial ports and with the recommended
CP2102 USB to serial converter.  Note that many
USB to serial converters won't go down to 45 baud,
and even that one has to be reprogrammed to go there.
If you need the programming utilities, I have them on line
at http://www.aetherltd.com/connectingusb.html
SiLabs no longer has them on line.

    The thick-film resistor is a stock item on Digi-Key.
Order TA810PW2K00JE-ND, $4.71 each.  That's an unusual
resistor.  I chose it because a typical 2K 10W resistor
installed directly on a PC board, will overheat the board
material.  The upright thick-film design does not dump
its heat into the board.  There are cheaper resistors, but
they're not all suited for direct board mount.

    Someone really should design a more modern board.  I was
working on one, but didn't finish it.  I wanted to
have a boost converter to generate 120VDC during
SPACE, to charge a capacitor.  On a SPACE to
MARK transition, the capacitor would discharge
into the selector magnet at 60mA. The timing on
the boost converter would then change to produce
only 13V, to hold a 220 ohm selector magnet at 60ma.
Almost all the power would be used, rather than dissipated
in a ballast resistor, reducing the load to what a USB
port could power.

			John Nagle


On 02/16/2015 04:58 PM, greenkeys-request at mailman.qth.net wrote:
> Message: 2
> Date: Mon, 16 Feb 2015 10:33:27 -0600
> From: Jeffrey D Angus<jdangus at att.net>
> To:greenkeys at mailman.qth.net
> Subject: Re: [GreenKeys] RS232 to TTY Loop  Boards
> Message-ID:<54E21BD7.5080000 at att.net>
> Content-Type: text/plain; charset=windows-1252; format=flowed
>
> Perhaps I wasn't quite clear on this.
> I'm NOT making the boards.
> I_am_  offering to assemble them if you don't want to make them yourself.
> Labor to assemble them is roughly $10 each + shipping.
>
> However, that being said, If I get enough orders for boards, I can have
> them
> made via ExpressPCB. They are, not cheap. For example, a 1.9" x 2.2" board,
> double sided with solder mask and silk screen is roughly $300 for twenty of
> them. So $15 apiece.
> Without the silk screen and solder mask, the cost drops down to $153 or
> about $7.65 each.
> The "set up cost per order" is the real cost here. $245 for the full
> board, and
> $61 for the one without the solder mask and silk screening.
>
> Obviously, if you all go crazy and order enough of them, it brings the total
> distributed costs down. For example, 40 of the simple boards drops down to
> $5.85 apiece instead of $7.65.
>
> Then, there's also the cost of the parts for the boards. Mouser does give a
> price break on quantity. So that helps.
>
> However, both the PS7141E and the thick film 10 watt resistor are non-
> stocked items. The cost of them are about $3 each.
>
> If y'all want me to, I will sit down, do a PC board layout for currently
> available parts, and an itemized parts list with costs for all three items.
> Parts, PC board and Labor. My guess at this point is going to be around
> $30 per board assembled. Closer to $40 if you want the pretty boards
> with solder mask and silk screen.
>
>
>
> -- Jeff-1.0 wa6fwi http://www.foxsmercantile.com