The Sinclair QL Forum

Do any of you know the maximum current draw the serial ports on the Ql can handle?

Basically I'm looking at using the 12v supply to drive the Composite Video/RGB switching on a SCART connector (pin 16) which is specified as 3v, up to 20mA.

I s'pose with big enough resistors in the voltage divider circuit it shouldn't be a problem, but I wanted to check first.

According to the schematics, the +12V line fed to the serial port connectors goes via a 680 ohm resistor. The maximum current into a short circuit would be 17.6mA.

If the SCART pin 16 on the TV has an input resistance of 75 ohms, the current would be 15.89mA with 1.19V being developed across this 75 ohm resistor. This voltage should be just okay to switch most TVs to RGB input mode. This 15.89mA current should not be a problem for the QL.

Note that because of the 680 ohm resistor in the QL, if you do use the serial port +12V line for this, you cannot use it for anything else.

See info here

Mark

Thanks!