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Old 26th September, 2002, 08:46 PM
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Question help plz

whats the maximal current i can send trough a COM-Port at standard-mainboards??
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Old 26th September, 2002, 09:16 PM
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I'm not sure why you're asking the maximum current. A com port is voltage sensitive, not current sensitive. The RS232 spec allows for a maximum of + and - 30V. The minimum allowed is + and - 3V. I'd not recommend going too close to the maximum, as I don't know how good the transceiver chips on your motherboard is.

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Old 26th September, 2002, 09:26 PM
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i need to know it for a project for school.i wanna support a circuit with a ┬Ácontroller and resistors and some things trough the COM-port.
or maybe someone knows a site with informations on that topic.
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Old 27th September, 2002, 11:36 AM
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If you want to interface something to the COM port, then...

If you've using TTL logic (5V), you can get a transceiver that will act as a level converter in both directions. That leaves you to worry about TTL levels instead of RS232 levels.

If you're using CMOS (3.3V), you can also get a transceiver.

If you're doing your own analog stuff, then you'll have to do your own level shifter.

On the RS232 receive lines, don't worry about how much current you put into them. Worry about the voltage instead! As long as your voltage is within +25 to -25v, there will be no damage. Remember, an RS232 receive is VOLTAGE sensitive, not current sensitive.

You can consider the RS232 receive lines to be equivalent of a resistor, with resistance between 3000 and 7000 ohms. Typically it's around the 5KOhm mark.

On RS232, Mark and Space are represented as a negative voltage and a positive voltage respectively. You need to manage at least -3V and +3V in order to communicate properly. (Technically you're meant to use at least -5v and +5v). If you're between -3V and +3V, there's no telling what state the port will think the line is in.

On RS232, an idle line is pulled to Mark (-ve) to indicate it's idle.

The transmit lines probably won't be able to source much more than 12mA. Some ports won't even be able to do that, perhaps as little as 6mA. Occasionally you'll come across a port that'll shut itself down if you draw too much current.

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