Q: The user guide for the USB-2500 series states that each digital pin can supply up to 1 mA and for applications that require more current an external 9 volt power supply can be used. What is the maximum current when using an external 9 volt power supply?

A: That statement can be a bit confusing and deserves an explanation. The outputs are true TTL 3v logic. There is no current driver capabilities for driving relays or other high current loads. Their purpose is to interface to another TTL circuit. The reason for this is when just using USB power you can pull about 1mA. When adding a 9V external power supply you can expect up to 10 mA. The issue is that there is a 220 ohm resistor in series with the outputs. So at 10mA your voltage will only reach about 1 volt. Basically, the voltage drops as the current increases and vice versa.