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Author Topic: Max output current from a single Parallel port pin?  (Read 3867 times)

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Offline Sweep

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Max output current from a single Parallel port pin?
« on: September 18, 2012, 03:23:48 PM »
I am wanting to build a simple circuit board with connection blocks and resistors for the 13 input pins on my second parallel pin port.
I had planned that the individual circuits would be very basic and similar to that shown on page 4-8 of the Using Mach3 Mill instruction book.
This board will be positioned in my control cabinet and allow easy connection to the parallel pin port of the computer and various switching devices for the input signals. The switching devices will likeley be a mixture of relay contacts, rocker switches, push buttons, and the connections of a MPG.
I would like to use one of the four output pins (1,14,16 or 17) of the same board for the 5V supply but I am unsure of how much current I can draw from one pin which, I guess, will determine the size of my resistors, and is there a maximum size of resistor that can be used for each input individual input circuit before the voltage drops too low to change its state? i.e. can I use, say, 10kOhm on each leg to keep down the current of the output pin?
The simple circuit on page 4-8 uses 470ohm resistors but, in the unlikely event of all 13 switches being closed at the same time, can the single input supply sufficient current (138mA) without a problem?

thanks ...Sweep

Offline Tweakie.CNC

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Re: Max output current from a single Parallel port pin?
« Reply #1 on: September 19, 2012, 01:45:29 AM »
Hi Sweep,

Obviously not all parallel ports are produced to the same specification but the maximum value that can safely be drawn from any one pin should be considered as not to exceed 10mA (the port is primarily designed to sink current rather than source it).

Just as a tip, it would be far better to use a USB port for the source of the 5 Volts supply in your circuit (or else use a separate 5 Volt supply).