Commands for sending TTL Voltages out of serial port?

[Beginner711]

In MAX, (or labview) I would like to send a TTL logic voltage to a pin.  I want to put 5 volts on a pin and be able to toggle it on and off with commands.  Also, is there a way to find out which serial port pin is the ground?

thanks,

shawn

[Buechsenschuetz]

Maybe it is possible to toggle one of the control lines of a serial port by software. I think the main problem is that the output voltage of a serial port is specified between +5 and +12V for a logic high and between -5V and -12V for a logic low. So it will not be possible to get TTL levels (0 to 0.8V for logic low and 2.3V to 5.0V for a logic high) out of a serial port without modifying the driver hardware.

[Dennis_Knutson]

There's a tutorial on using the parallel port <A href="http://zone.ni.com/devzone/conceptd.nsf/webmain/72C6FC6CE4AD4D1386256B1800794596">here</A>. As mentioned, the serial port is not even close to TTL.

[Beginner711]

Dennis,

When I toggle the pins high and low with the parallel port vi that I got from the tutorial link you give, the pins go from roughly 0 volts to .26 volts, which isn't exactly TTL logic levels.  At least not for the high value.  I have grounded pins 11 and 12 as it says in the tutorial.

Should I be able to get a proper "high" voltage of above 2.3 volts for TTL logic?

Curious if I've done something wrong.

thanks!

shawn

[Buechsenschuetz]

I am not very familiar with the hardware configuration of a printer port. But from the results you get I would guess that you are working with (one of) the pin(s) which has an open-collector circuit.

Try to connect the output pin you are working with to +5V (maybe an external supply but it must have common ground with the host PC) via a resistor with, say, around 10kOhm, and measure the levels again.