Problem interfacing to the LPT Port

[dynoboy]

Hi Can anyone help ME!!!

I am trying to Interface a FPGA to the LPT Port of my computer. I am planning to use the Data Register (Pins 2 - 9).
The interfacing circuitry I am using comprises of only a Buffer IC and a resistor. I am using the 74125 IC, (its a Tri state CMOS Quad Buffer, I have forced the control pins to 0, i.e. its active state).
Since I require 8 pins I am using 2 such IC's. The circuit used is as such:

FPGA Pin> --^v^v^v---I >--------------------<LPT Port Pin
_________1K ohm __Buffer (1/4 74125)

As long as the LPT port is NOT connected the voltage at the output pins is 3.3V, but as soon as I CONNECT the o/p pin to the LPT Port the voltage drops to 2.02V. The reason is not clear even a 1K pullup resistor cannot help my case.

Please Help Me if any one possibly can...I need to submit my project next week.

Thanks a bunch,

 

[klug]

You need to set LPT port in READ mode. Seems in your case, that it is in output mode and 0's are in data pins.

Also not all LPT ports can be configured to READ through data pins. If you have old MB, or some installed old LPT card, then there is a chance that this port can not be uset to read through data pins.

To switch LPT port to read state you need to set 5th configuration bit in LPT_address_port+2. If your port address is 378h then you need set 5th bit in 37Ah port.

Also, do not forget to set correct mode for LPT in your bios if you are using LPT from MB.

 

[dynoboy]

Hi klug,
thanks for your reply. But it seems that you have not got my question right. The problem is that there is a drop in the Voltage levels, that could mean that the device is not being able to handle that much current.
Also if i remove say 7 (out of the total 8 pins) the voltage at the pin increases to around 3V.

So its not the Operational Mode of the LPT Port output pins, it seems that my buffer is not able to handle, so its the circuit porblem.

Awaiting your response.
Thanks

 

[tlp71@hotmail.com]

maybe that you have a gnd problem.
Try to interconnect GND of two boards, in fact if you have not the same gnd your voltage tension is relative, not absolute.
All voltage are related our GND.
I hope that this can help you.
Bye.

 

[dynoboy]

Hi,
There is no problem with the Ground, The ground of the FPGA and the Computer are connected. The Power Supply for the Buffer Card is the one the FPGA uses so infact all teh grounds are connected.