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From: miso on 6 Jan 2010 17:32
On Jan 6, 4:54 am, lighthouseman <ke...(a)kmillar.co.uk> wrote:
> Here's a nice simple one for you....
> If my design has 5 CMOS serial ports, and I use something like 3 x
> MAX232, which have 2 drivers/receivers on each chip, then what should
> I do with the unused inputs/outputs on the 3rd chip?
>
> Should I leave the CMOS inputs floating or tie them up or down?
> What about the RS232 inputs? Should I leave them floating or tie them
> to VSS or VCC?

First thing you should consider is you often don't even need 232
chips, especially for short connections. While the spec allows the
receiver trip point to be between -3V to +3V, and technically a 0V
trip point would be optimal, most receivers trip at a bit over a volt.
Thus you can drive 232 receivers with CMOS. You only need to go
between the supply rail and ground.

Now rolling your own receivers is another story, but basically it is
done with voltage dividers, a voltage clamp, and CMOS inverters. If
you traced out the receiver used in most 232 devices, that would be
what you find.

Receivers have a resistor built in, pulling them to ground. You would
need to consult the data sheet regarding transmitter inputs. If the
device has pull ups on the transmitter input, then float it, otherwise
ground it. [Grounding pins pulled up wastes power, but you knew
that. ;-]

The reason I mention you don't really need a 232 chip is every 232
chip on the market adds jitter to the signal. It is hard to keep the
charge pump noise out of the receivers and even transmitters to some
degree. I've measured jitter such chips (well, my designs) and the
peak to peak is about 1uS. Not a big deal in 232, but something to
think about. I don't think any of the data sheets spec the jitter.

There are some 232 type chips with a negative trip point on the
receiver. This was used in AutoShutDown mode. It should be on a
handshake line, not data. [I can't possibly know ever variation of 232
out there, so I need to be a bit vague about this.]
From: Jan Panteltje on 6 Jan 2010 18:01
On a sunny day (Wed, 6 Jan 2010 14:32:54 -0800 (PST)) it happened
"miso(a)sushi.com" <miso(a)sushi.com> wrote in
<36580fbc-79ed-4e00-9d44-9c5326aa85ba(a)22g2000yqr.googlegroups.com>:

>The reason I mention you don't really need a 232 chip is every 232
>chip on the market adds jitter to the signal.

You can try a MAX-panteltje:
http://panteltje.com/panteltje/wap54g/max_pantel.txt
but then you need control over RTS and DTR.

Having the MAX232 is rather universal, and always works.








It is hard to keep the
>charge pump noise out of the receivers and even transmitters to some
>degree. I've measured jitter such chips (well, my designs) and the
>peak to peak is about 1uS. Not a big deal in 232, but something to
>think about. I don't think any of the data sheets spec the jitter.
>
>There are some 232 type chips with a negative trip point on the
>receiver. This was used in AutoShutDown mode. It should be on a
>handshake line, not data. [I can't possibly know ever variation of 232
>out there, so I need to be a bit vague about this.]
>
From: Jamie on 6 Jan 2010 20:19
lighthouseman wrote:

> Here's a nice simple one for you....
> If my design has 5 CMOS serial ports, and I use something like 3 x
> MAX232, which have 2 drivers/receivers on each chip, then what should
> I do with the unused inputs/outputs on the 3rd chip?
>
> Should I leave the CMOS inputs floating or tie them up or down?
> What about the RS232 inputs? Should I leave them floating or tie them
> to VSS or VCC?
tie the inputs to something.



From: miso on 7 Jan 2010 22:05
On Jan 6, 3:01 pm, Jan Panteltje <pNaonStpealm...(a)yahoo.com> wrote:
> On a sunny day (Wed, 6 Jan 2010 14:32:54 -0800 (PST)) it happened
> "m...(a)sushi.com" <m...(a)sushi.com> wrote in
> <36580fbc-79ed-4e00-9d44-9c5326aa8...(a)22g2000yqr.googlegroups.com>:
>
> >The reason I mention you don't really need a 232 chip is every 232
> >chip on the market adds jitter to the signal.
>
> You can try a MAX-panteltje:
>  http://panteltje.com/panteltje/wap54g/max_pantel.txt
> but then you need control over RTS and DTR.
>
> Having the MAX232 is rather universal, and always works.
>
>  It is hard to keep the
--------
Did the Maxim police cut you off at mid-sentence?

How do you insure the 1n4148 doesn't hog all the current, preventing
the NPN from turning on. Does this just depend on the ratio of
junction area between the diode and transistor?

There are devices that only need to receive 232. In that case, a home
brew receiver is fine. Rolling your own transmitter is another story.
There are cases where the output is shorted. You really can't
controller the external world. But a bit of resistance in series with
the output would do the trick.

I made a data slicer that didn't use a 232, though I've seen designs
swearing you need one. By skipping the 232 chip, I made the design
much easier to phantom power from the transmitter. If you ever do
this, shoot for single supply use since most of these 232 chips just
charge pump invert the boosted signal, so the negative supply is not
as beefy.

[Given your website and hobbies, I suspect you know what a data slicer
is.]

From: Jan Panteltje on 8 Jan 2010 09:23
On a sunny day (Thu, 7 Jan 2010 19:05:08 -0800 (PST)) it happened
"miso(a)sushi.com" <miso(a)sushi.com> wrote in
<7eab3bb5-dd81-4a6c-bca0-b3f1395858fb(a)j5g2000yqm.googlegroups.com>:

>On Jan 6, 3:01�pm, Jan Panteltje <pNaonStpealm...(a)yahoo.com> wrote:
>> On a sunny day (Wed, 6 Jan 2010 14:32:54 -0800 (PST)) it happened
>> "m...(a)sushi.com" <m...(a)sushi.com> wrote in
>> <36580fbc-79ed-4e00-9d44-9c5326aa8...(a)22g2000yqr.googlegroups.com>:
>>
>> >The reason I mention you don't really need a 232 chip is every 232
>> >chip on the market adds jitter to the signal.
>>
>> You can try a MAX-panteltje:
>> �http://panteltje.com/panteltje/wap54g/max_pantel.txt
>> but then you need control over RTS and DTR.
>>
>> Having the MAX232 is rather universal, and always works.
>>
>> �It is hard to keep the
>--------
>Did the Maxim police cut you off at mid-sentence?

That is a remaining line from your own posting, did you not recognise the text?

I use the MAX232 a lot too.
Just not the 3.3V version, and that only because it is not in the shops here.
I am not Joerg, who can only afford 2 cent transistors.
Poor guy ;-)

>How do you insure the 1n4148 doesn't hog all the current, preventing
>the NPN from turning on. Does this just depend on the ratio of
>junction area between the diode and transistor?

They are reverse connected,
the diode conducts when the input is negative, protecting the be junction from
too much revere voltage,
and the be junction conducts when the voltage is positive.


>There are devices that only need to receive 232. In that case, a home
>brew receiver is fine. Rolling your own transmitter is another story.
>There are cases where the output is shorted. You really can't
>controller the external world. But a bit of resistance in series with
>the output would do the trick.

In this case the current limit is set by the DTR and RTS lines...
So it is always in spec if i connects to a real RS232 port.


>I made a data slicer that didn't use a 232, though I've seen designs
>swearing you need one. By skipping the 232 chip, I made the design
>much easier to phantom power from the transmitter. If you ever do
>this, shoot for single supply use since most of these 232 chips just
>charge pump invert the boosted signal, so the negative supply is not
>as beefy.
>
>[Given your website and hobbies, I suspect you know what a data slicer
>is.]

Actually electronics is not only a hobby, also made a living with it.

To 'do the RS232 thing right' there are many ways.
In an other design I made negative with a simple sine wave oscillator,
rectifier and pot core.
That then also powers other stuff.
Creating a negative supply almost never is a problem,
here are some details of what is one directory level up:
The generator:
ftp://panteltje.com/pub/8052AH_BASIC_computer/dc_dc_converter_detail/5V_to_30V_converter.jpg
The EPROM programmer switch:
ftp://panteltje.com/pub/8052AH_BASIC_computer/dc_dc_converter_detail/EPROM_programming_voltage_generator.jpg
The RS232 interface:
ftp://panteltje.com/pub/8052AH_BASIC_computer/dc_dc_converter_detail/RS232_interface.jpg
Hope this helps:-)


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