RS-232 to LVTTL

[John Larkin]

On Mon, 11 Apr 2005 16:30:04 GMT, bok118@zonnet.nl (Gerard Bok) wrote:

On Mon, 11 Apr 2005 09:43:17 -0500, John Fields
jfields@austininstruments.com> wrote:

On Mon, 11 Apr 2005 12:30:15 +1200, Terry Given <my_name@ieee.org
wrote:

John Larkin wrote:
On Mon, 11 Apr 2005 11:51:01 +1200, Terry Given <my_name@ieee.org
wrote:


Mac wrote:

I want a quick and dirty conversion of RS-232 to LVTTL. That is, it is
TTL except that the signal cannot exceed ~3.3 V.

Will this work (use courier or similar for ASCII art schematic):

VCC (5V)
|
/
\ R2 (1k)
/
|
+----------------+-> to LVTTL device
| |
R1 (22k) /c /
RS-232->---/\/\/\/\--+----| Q1 (npn) \ R3 (1k)
| \e /
--- | |
D1 / \ | /
| | \ R4 (1k)
+--+--+ /
| |
GND GND

parallel a resistor with D1, to ensure the transistor stays off over
temperature with the RS232 disconnected (Icbo has to go somewhere).


But one could replace the diode with a resistor and make everybody
happy.

yep. I'd use a single 10k quad-pack, and have a 2V5 (cf 3V3) output.

---
RS-232 can go as high as (IIRC) +/- 15V,

It's even worse. Older versions allow for + / - 30 volts

and a B-E junction
reverse-biased at 7.5V isn't a good thing.

It would be nice to see it, though
IIRC you won't get far beyond - 5.5 volt

Zenering the b-e junction progressively degrades beta, although I
haven't seen (or measured) the magnitude of the effect.

John

 

[John Woodgate]

I read in sci.electronics.design that John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote (in
<afbl51lfouus6e4411o5hh3ou3o1pv5ium@4ax.com&gt about 'RS-232 to LVTTL',
on Mon, 11 Apr 2005:

Let's see if we can make a 500-post thread out of this circuit.

In another thread, we got from colloidal silver to blood, which is Gore,
so that draws in the Florida election. Now, can we get from that to
RS-232 or LVTTL?
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Jim Thompson]

On Mon, 11 Apr 2005 10:09:27 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Mon, 11 Apr 2005 16:30:04 GMT, bok118@zonnet.nl (Gerard Bok) wrote:

On Mon, 11 Apr 2005 09:43:17 -0500, John Fields
jfields@austininstruments.com> wrote:

On Mon, 11 Apr 2005 12:30:15 +1200, Terry Given <my_name@ieee.org
wrote:

John Larkin wrote:
On Mon, 11 Apr 2005 11:51:01 +1200, Terry Given <my_name@ieee.org
wrote:


Mac wrote:

I want a quick and dirty conversion of RS-232 to LVTTL. That is, it is
TTL except that the signal cannot exceed ~3.3 V.

Will this work (use courier or similar for ASCII art schematic):

VCC (5V)
|
/
\ R2 (1k)
/
|
+----------------+-> to LVTTL device
| |
R1 (22k) /c /
RS-232->---/\/\/\/\--+----| Q1 (npn) \ R3 (1k)
| \e /
--- | |
D1 / \ | /
| | \ R4 (1k)
+--+--+ /
| |
GND GND

parallel a resistor with D1, to ensure the transistor stays off over
temperature with the RS232 disconnected (Icbo has to go somewhere).


But one could replace the diode with a resistor and make everybody
happy.

yep. I'd use a single 10k quad-pack, and have a 2V5 (cf 3V3) output.

---
RS-232 can go as high as (IIRC) +/- 15V,

It's even worse. Older versions allow for + / - 30 volts

and a B-E junction
reverse-biased at 7.5V isn't a good thing.

It would be nice to see it, though
IIRC you won't get far beyond - 5.5 volt


Zenering the b-e junction progressively degrades beta, although I
haven't seen (or measured) the magnitude of the effect.

John

About a week under the hood of a car completely kills the transistor
action. Sometimes it also kills the "Zener" effect as well.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[Mac]

On Mon, 11 Apr 2005 16:30:04 +0000, Gerard Bok wrote:

On Mon, 11 Apr 2005 09:43:17 -0500, John Fields
jfields@austininstruments.com> wrote:

On Mon, 11 Apr 2005 12:30:15 +1200, Terry Given <my_name@ieee.org
wrote:

John Larkin wrote:
On Mon, 11 Apr 2005 11:51:01 +1200, Terry Given <my_name@ieee.org
wrote:


Mac wrote:

I want a quick and dirty conversion of RS-232 to LVTTL. That is, it is
TTL except that the signal cannot exceed ~3.3 V.

Will this work (use courier or similar for ASCII art schematic):

VCC (5V)
|
/
\ R2 (1k)
/
|
+----------------+-> to LVTTL device
| |
R1 (22k) /c /
RS-232->---/\/\/\/\--+----| Q1 (npn) \ R3 (1k)
| \e /
--- | |
D1 / \ | /
| | \ R4 (1k)
+--+--+ /
| |
GND GND

parallel a resistor with D1, to ensure the transistor stays off over
temperature with the RS232 disconnected (Icbo has to go somewhere).


But one could replace the diode with a resistor and make everybody
happy.

yep. I'd use a single 10k quad-pack, and have a 2V5 (cf 3V3) output.

---
RS-232 can go as high as (IIRC) +/- 15V,

It's even worse. Older versions allow for + / - 30 volts

and a B-E junction
reverse-biased at 7.5V isn't a good thing.

It would be nice to see it, though
IIRC you won't get far beyond - 5.5 volt

FWIW, the mmbt2222a and the 2n3904 data sheets both list Vebo max at 6
Volts.

--Mac

 

[Mac]

On Mon, 11 Apr 2005 03:57:53 +0000, Si Ballenger wrote:

On Mon, 11 Apr 2005 02:46:49 GMT, Mac <foo@bar.net> wrote:


I'd make that part of the circuit and check it out with a meter
before connecting it to a chip to see is it works asw desired.

I'm going to make the whole circuit, connect up the input and power,
and check the output on an oscilloscope. But what I really want to
know is, do you have some articulable reason to believe that the
circuit won't work? If so please state it!

All I can say is I haven't tried it. How about you trying it and
report the results.

If that is all you can say, then why join the fray at all?

It's not like Voltage dividers or transistor switches are black magic or
something.

Then why all the questions???? ;-)

I really only had one question, and it was answered satisfactorily by two
people within just a short time.

Why did I ask? To benefit from the experience of others, of course. I've
designed transistor switch circuits many times on circuit boards, but I've
never done an RS-232 converter. Usually when interfacing RS-232 to logic,
I have used a transceiver. But in this case, for various reasons, I don't
want to.

As it turns out, I did receive a useful suggestion from John Larkin.
He pointed out that I can't count on high voltage drive from some wimpy
RS-232 sources (e.g., laptops), so I probably should be more conservative
on my overdrive calculation.

After that, in most of my posts I was just trying to be polite, and in
some cases to lead other people to enlightenment.

regards,
Mac

 

[John Larkin]

On Mon, 11 Apr 2005 12:30:15 +1200, Terry Given <my_name@ieee.org>
wrote:

John Larkin wrote:
On Mon, 11 Apr 2005 11:51:01 +1200, Terry Given <my_name@ieee.org
wrote:


Mac wrote:

I want a quick and dirty conversion of RS-232 to LVTTL. That is, it is
TTL except that the signal cannot exceed ~3.3 V.

Will this work (use courier or similar for ASCII art schematic):

VCC (5V)
|
/
\ R2 (1k)
/
|
+----------------+-> to LVTTL device
| |
R1 (22k) /c /
RS-232->---/\/\/\/\--+----| Q1 (npn) \ R3 (1k)
| \e /
--- | |
D1 / \ | /
| | \ R4 (1k)
+--+--+ /
| |
GND GND

Thanks!

--Mac


parallel a resistor with D1, to ensure the transistor stays off over
temperature with the RS232 disconnected (Icbo has to go somewhere).

Cheers
Terry


But transistors don't leak any more. I bet the diode leaks more than
the transistor does!

A BC847BPN is spec'd at Icbo = 5uA at Tj=150C. 15nA typ.

And how many RS-232 receivers commonly work at 150C? Even at 5 uA, it
would still deliver a perfectly good high. And who cares what the
logic level is when nothing's connected?

I recently picked up a copy of RF Shea's "Transistor Circuit
Engineering" (its really a GE book), for the whopping price of NZ$0.50.
The chapter on bias stability discusses using leaky diodes to
compensate for leaky transistors over time, temperature and sock colour.

You wear sox in NZ?


Let's see if we can make a 500-post thread out of this circuit.

John