Originally Posted by
CNCMAN172
As has been noted many BOB do require an external power supply depending on the features found on the boards. Some require no added supply, most of those are older very basic models. Here is an example of a good BOB with a manual, many Chinese units from ebay come with no manuals and no schematics. Look at page 6 in the attached manual and you will see they schematic of the interface that shows the pull up resistors.
Your current parallel port does not have pull up resistors, so the outputs are from open collector transistors. If you measure the voltage at the output pins like pins 2 and 3, when not being activated they will measure a TTL level high about 3.6 to 3.8 volts. However, if you attach a load to those outputs the voltage will fall drastically because there is no pull up holding the collector high and so there is no path for the current to flow. So under load the voltage will actually be less than 2 volts. The reason the pulses coming from Mach3 will allow you to spin a motor is they are pulling the outputs low and the open collector goes up just enough to allow it to work. However, when you try to use direction signal it works in one direction when it pulls the output low, but the high will not work because the current can not flow through the collector of the transistor when requested high.
Russ