Low current logic to RS232 conversion

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Hi, I need to generate an RS232 signal from a TTL line. + & - 12v supplies
available, and the setup must be low current draw. Currently I'm using 8 pin
DIP SN75155s, neet footprint but the quiescent current drain is over 8mA
which is too high here. 1488s are even higher. The alternative seems to be
to use discretes, but with my limited design skills I keep ending up with
what seems to be a ridiculously high component count - 4+ semis and 6+
resistors by the time I've managed to drive from both rails. Surely there's
a neat way of doing this that also keeps the current draw right down
(ideally < 1mA on either supply with no imposed load). Anyone have a
suggestion? TIA



Re: Low current logic to RS232 conversion
On Tue, 19 Aug 2008 19:48:29 +0800, "Bruce Varley"

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Here is a design for an RS232-to-5V interface cable between a Casio
diary and a standard RS232 device:
http://web.archive.org/web/19991004085223/http://pfranc.com/pclink/myidea.htm

The design is powered from the RS232 interface pins, so I assume the
current draw is low. The author states that several components can be
omitted, so you can probably get away with a single FET, zener, and
four resistors.

Otherwise have a look at Maxim's line of RS232 chips.

- Franc Zabkar
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Re: Low current logic to RS232 conversion
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Bruce,
have a look at:
http://www.dontronics-shop.com/990008-xbee-serial-adapter-rs232-ttl.html

looks like 5V@.5ma
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2221
Max3238e, check it out and make sure I didn't read it wrong.

Cheers Don...



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Don McKenzie

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Re: Low current logic to RS232 conversion
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The MAX3222 comes to mind, takes around 0.3mA supply current.
No need for the +/-12V rails, generates it's own (double the supply
voltage, you don't actually need 12V).
Maxim probably have even lower power devices than that too.

Dave.

Re: Low current logic to RS232 conversion
I know I'm stating the obvious but have you tried using a humble opamp?

*Peter*

Bruce Varley wrote:
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