really simple RS232-TTL converter... does it work?

[Andrew Holme]

Gotcha. So you enable hardware handshaking, thereby driving RTS and
DTR positive, and draw positive supply from it? How much current can
they source?

You can only take a few milliamps. The source impedance is quite high,
typically 1K, so the voltage falls by (e.g.) 1V for every milliamp drawn.

One thing I still don't get: since RS232 is a full-duplex protocol, how
can you derive a negative voltage from TX? When the computer isn't
sending data, I understand that TX will idle at -12V, but isn't there a
possibility that it will be switching between +12 and -12V rapidly
when you're trying to use it as your negative supply?

RS232 can be half-duplex or full-duplex.

You need to use diodes and smoothing capacitors on all the supply inputs.
When the transmit line is high, the diode switches-off, and the capacitor
provides the negative supply.



DTR ->|---.
|
|
RTS ->|---o-o----- +ve supply
|
|+
=== 10uF
/-\
|
|
===
GND


Tx --|<----o------ -ve supply
|
|
\-/
=== 10uF
+|
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

 

[Andrew Holme]

I better give you the full circuit:





DTR ->|---.
| ____
| | 78 |
RTS ->|---o-o--------| L05|----o---o +5V
| |____| |
|+ | |
=== 100uF | --- 100n
/-\ === ---
| GND |
| |
=== ===
GND GND


Tx --|<----o------ -ve supply
|
|
\-/
=== 100uF
+|
|
===
GND

(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

 

[Jim Thompson]

[email protected] wrote:
Unless you are driving a long line, I would avoid any 232 chip with a
charge pump. They add jitter to the signal. [I've designed some of the
Maxim parts, and trust me, it is something you check in the lab since
the test engineer needs to know the limits of the jitter. I don't know
if any datasheets show this.]

Wow, you designed those? Cool!

The typical 232 receiver is just an inverter, a resistor divider, and a
diode clamp. The -3V trip points doesn't have to be reached as the trip
point of the inverter is above ground.

Thanks, that's very good to know. So if I have this right, most
computers just use a circuit like this as an RS232 receiver?

RS232
LEVEL
INPUT
*------------+
|

< R1 TTL
LEVEL
| |\ OUTPUT
+----------+------| |O-----*
| | |/
---
< R2 / \
|
| |
/// ///

If so, then I can certainly see how 0V/+12V works as "RS232" signaling.
Why does RS232 use such goofy logic levels anyway?

Dan

Naaah!

More like....

RS232
LEVEL
INPUT
*------------+
|

'HCT Inverter < R1 TTL
LEVEL

| |\ OUTPUT
+-----------------| |O-----*
|/


The MC1489 (my design) has a (floating-tub) input resistor into the
base of an NPN transistor, with a large-value feedback resistor from
output to provide a modicum of hysteresis.

...Jim Thompson

Actually, the other circuit is closer to what was used. Just going to
the inverter gate with a resistor would cause the impedance to be very
high as the input went between ground and the positive supply rail.

The divider in the Maxim designs was needed to keep the gate voltage
within limits. Actually, TI started to use those cheapie receivers,
and we just copied them. The parts with automatic shutdown have an
extra receiver that can sense if the input is at ground. The auto
shutdown plus was a bit different, and I think really only done to
prevent someone else from doing the design, i.e. create some prior art.


If you sweep the input of the 232 chip, you should be able to find the
kink in the impedance when you go beneath ground

The 1489 is _bipolar_ so there's essentially back-to-back diodes to
ground.

I'd add a resistor from "RS232 Level Input" to ground, rather than
from the inverter input... otherwise you lose noise immunity.

...Jim Thompson

 

[Ken Smith]

Yup. In fact, 0 and 5 has worked fine on every computer I've ever used,
including serial ports built in on motherboards, add-on PCI cards,
laptops, and USB to RS232 converters. It's very convenient. It didn't
work on a Palm handheld once; that required a proper negative voltage.

It doesn't work on quite a few laptops. I think one of the low power
RS-232 chips has the zero/one boundary below ground.

 

[vasile]

Take a look here too:

http://surducan.netfirms.com/RS232.html

Withe the version4 of the schematics presented here, using a CAT5 cable
(Tx-ground, RX-ground, etc) I was able to transmit data at 9600bps 8N1,
over 100m with zero errors with the following module:
http://surducan.netfirms.com/module.html

So it works good and you may grab from almost all RS232 up to 10-15mA.
But if you plan such communications outside, better use an optoisolated
RS232.

greetings,
Vasile

 

[[email protected]]

Jim Thompson skrev:

I don't anything anymore even puts out ±12V.

To be totally Kosher I'd add Ge diodes from the inverter input to each
rail to keep current out of the ESD diodes on the chip.

snip

Is it not ok to push current through the ESD diodes as long as you stay
below
the maximum specified in datasheet? (and make sure you use at least
that
current on the rail)

I belive Xilinx specifies some of their fpga inputs for max voltage and
max current
and you can pick what ever is easiest

-Lasse

 

[Jim Thompson]

Jim Thompson skrev:
snip

Is it not ok to push current through the ESD diodes as long as you stay
below
the maximum specified in datasheet? (and make sure you use at least
that
current on the rail)

I belive Xilinx specifies some of their fpga inputs for max voltage and
max current
and you can pick what ever is easiest

-Lasse

I don't want to re-start the old argument, but...

I personally never run signal currents thru ESD clamps.

Others do.

Do whatever you want.

Someday your purchasing department will substitute a Chink chip and
your circuit will go up in flames on a customer's site.

I have been working with <major Chink semiconductor company that shall
remain nameless> that literally stole a process from a reputable
Taiwan company.

Chink company can't find asshole using both hands, flashlight and a
mirror, let alone cope with the ART of designing ESD structures.

Buyer beware ;-)

...Jim Thompson

 

[jasen]

Yup. In fact, 0 and 5 has worked fine on every computer I've ever used,
including serial ports built in on motherboards, add-on PCI cards,
laptops, and USB to RS232 converters. It's very convenient. It didn't
work on a Palm handheld once; that required a proper negative voltage.

most of them use the MC1489 receiver chip, and as Jim will tell you
anything below about +2V is considered as a low by that chip.

I suppose it depends what you want it for. Obviously you don't want to
send something like that to a customer, but for you own use it will very
likely be adequate.

yeah, if you want to send the signal through a mile of doorbell wire having
+/- 12V signals will help.

If it's going across the desktop in a shielded cable +5/0V will be fine.

Bye.
Jasen