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RS232 max current

Posted on 2000-04-28
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Last Modified: 2008-02-26
How many Ampere can i have from the DTR and RTS pins of a RS232?

(this problem is related to the Nokia Data suite serial cable, you can find a scheme at
http://gsm-cables.com/nokia/cables.html.
I'v build the cable, but when i plug it in the COM, i have only 4.8V on the RTS and DTR wires. I think i ask too much current)
100 pts bonus if you can solve my cable problem!
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Question by:parduz
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vikiing earned 150 total points
ID: 2762055
>>i have only 4.8V on the RTS and DTR wires. I think i ask too much current)

You're talking of voltage, which has nothing to see with current.

There are some brands/models of tablets/digitizer which are fed directly from serial port.

Typical UART (the RS-232 I/O chip) allows up to 50 mA per output pin, so if your device needs 70 mA to run you will need to use at least 2 pins for power.

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Author Comment

by:parduz
ID: 2762759
Ok, i'm not an expert, so before accept your answer i ask you some clarifies:
A friend of mine answer to my question in this manner:
- You ask too many current, so the RS232 "sit-down" (?)-
So i have reported my doubts here.
What happens if i ask more of 50mA to a pin? The voltage stay at 11~12V? Or the output "burns"?
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by:vikiing
ID: 2763956
>>What happens if i ask more of 50mA to a pin?

That depends on HOW MUCH you want to extract; it's not the same to absorb 51 mA than 500.

If you start to suck a bit more, voltage may get reduced; if you suck MUCH more, it's possible output gets fried.
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Author Comment

by:parduz
ID: 2764702
OOOk!
So, if my circuit works good with a 12Vcc stabilized power supply (i can measure 12V on the "input" of my circuit), and don't work when connected to the RS232 port (as i say, i measure 4.8V on the "input" of my circuit), the cause can be that i ask too many current?
Why you say "You're talking of voltage, which has nothing to see with current"?


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Expert Comment

by:vikiing
ID: 2765601
>>as i say, i measure 4.8V on the "input" of my circuit), the cause can
>>be that i ask too many current?

I think it is; if you drain more current than source can supply, the effect is that voltage drops down.


>>Why you say "You're talking of voltage, which has nothing to see with
>>current"?

Because they are different magnitudes and measure different things, as if we were speaking of litres and kilos.

Amperage measures THE AMOUNT of energy that flows thru a circuit, whereas voltage indicates THE FORCE that current goes (grossly speaking).

The circulating current will depend on the resistance you load to that circuit. If you have a power supply, lets say, of 10 V, and you put a 1-Ohm resistor, the current will be of 10 A (according to Ohm's law); if that resistor is 10 Ohm, then current will be 1 A (under the same 10 V).

As you can see, the voltage was always the same: 10V, but current varied with resistance.

Of course, if you increase the voltage, the same will occur with current (as they're directly proportional), but that DOES NOT indicate that both magnitudes are the same.

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Expert Comment

by:mark2150
ID: 2765752
If the voltage is dropping to under +5 then you're definately overloading the driver. Typically you shouldn't pull more than 20mA from the line.

What is probably happening is that you're overloading the +12 line and it's sagging down to where the +5 starts supplying current and that's why the voltage is hanging around 5V.

The +12VDC line can only supply a limited amount of current. When you attempt to draw more current than it is capable of supplying the voltage will begin to drop until the current demand can be met.

There's enough to drive an LED, or to power a small CMOS circuit, but 100mA is way too much to draw.

RS-232-C drivers are designed to tolerate a variety of overload conditions (look up the specs on the MC1488 / MC1489) which is why you haven't fried anything. You can toss a dead short on the lines and they'll go into shutdown instead of dying. About the only thing that'll kill them is overvoltage beyoned +-15VDC or so.

M
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Author Comment

by:parduz
ID: 2767418
Hi, Mark.
It is a big time from last "encounter" at EE, how are you?
As usual, your comments are very useful.

Vikiing:
i understand what you says: my shool was
"Electrotecnical" (that means i studied BIG voltage and BIG amperage problems, triphases motors, transformers, and others stuffs like "how light a route", "how many kind of electrical plants we have", etc...), but my problem is i don't know nothing about diodes (shottky, zener, etc...), and other little object like these.
So, building a circuit that is found on variuos sites on the net, and finding problems, i'm lost in a lot of strange things.

For all:
Regarding the question, i'm ready to give pts at Vikiing.
But if someone like help me with this little circuit, i keep the question open to increase the premium with the bonus pts.

Good night to all. (Here is 23:35)
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LVL 12

Expert Comment

by:mark2150
ID: 2767688
I'm well.

Let me know what you need to know about diodes and such.

M
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