# RS232 output current rating

#### praondevou

Joined Jul 9, 2011
2,942
What is the output current rating for RS232 PC port signals?

It seems to be current limited (to avoid damaging it I guess) but I don't find specs for that.

#### thatoneguy

Joined Feb 19, 2009
6,359
Maxim's RS-232 Bible App Note Only mentions line capacitance and voltage levels, no max current.

RS-232 Spy cable

States RS-232 must be capable of driving 45mA
. We assume the attached device is in an idle state. Therefore, the voltage at line 2 is something like -12 Volt, while at the other end of the resistor +12 Volt is applied. Simple mathematics learns that a current of approximately 11 mA (=24 Volt/2200 Ohm) flows through the resistor. This is no problem because most RS232 driver IC's are capable to deliver at least 45 mA.
The 16550 UART could only sink 15mA, and used open collector outputs, it was the de-facto standard RS-232 UART, but hid behind a line driver, which, of course, varied.

#### praondevou

Joined Jul 9, 2011
2,942
Ok that's 10x more than I need.

Thanks

#### THE_RB

Joined Feb 11, 2008
5,438
All the early M$mouse type PC mice were driven and powered from the PC serial port (inlcuding the 2 LEDs used in the mouse optos to detect ball movement). They often used diodes to the RTS and DTR pins in the serial port to get a few more mA from each pin. Most of the pins in PC serial ports are 270 or 330 ohm output impedance and drive about 10v so if you don't mind loading them down to about 5v you can get maybe (ie best case) 15mA from each pin. Generally M$ mice would draw less that 20mA total from the combination of serial port pins.

Note- your PC software can set DTR and RTS pins to HI (+ve) to enable them to provide power to the target device.

You might want to read the Wiki on "M\$ mouse".