[lcd4linux] Carputer I/O: 2 displays and 16 GPIOs on a single connector?

[Zach W. <zwh...@da...>]

Hello Everyone,

I'm looking for some advice on my current project. I apologize for the
long post, but I want to make sure my assumptions are correct as well,
so I'm being a bit more verbose.

I'm currently working on my carputer project. My plan involves gutting a
2 DIN factory radio from my car and building a carputer into the
empty shell. Since I plan to have a 7" lcd mounted on my dash, I want
to use either a 20x2+24x2 or a 20x4 display in the radio case itself.

I want to make use of the existing buttons in the face, 6 of which are
lighted buttons which I will be toggling through the use of GPOs or
similar. I will have at least 6 more indicator lights which I would
like to toggle through GPOs, plus possible potentiometer and/or switch
inputs.

If I can, I'd like to simplify the interface between the computer and
the entire front display panel so I'm using only one connection between
computer and front panel. This probably won't happen, but it's still
one of my goals.

My current idea involves building something around two HD44780s. If I 
understand everything correctly, there are 5 pins available on the 
parallel port that can be switched high/low, allowing us to address 5 
devices on the same bus. Two would be used for the HD44780s, leaving 1 
for backlight control, and two left available.

I want to use two 74DC573N ICs to give myself a total of 16 GPIOs. 16
should be plenty for what I'm doing, and on a hardware level I don't see
why this wouldn't work. Can someone verify that what I want to solder up
is possible?

This also leaves me with no way to do keypad input. However, it seems
like the displays that have supported keypads do not have any GPIOs.
I can build something outside of lcd4linux to handle that, but if anyone
knows of a display with keypad pins (supported or not, I can hack in
support) and a large number of GPOs I'd love to hear about it.

This leaves the software part. From what I can tell, it seems that 
lcd4linux is limited to addressing a single GPIO per parallel port.
Would the ability to address more than 8 GPIOs be something that
interests anyone else? If so I'll post it as a feature request.

Finally, is there some other way to do all this I might have missed?
It seems like I can either have a keypad, or I can have a large number
of GPIOs. Given that choice I'll go for the GPIOs.

-Zach