Re: [lcd4linux] Presence detection of HD44780 display?

[Michael R. <re...@eu...>]

Hi Reinhard,

> In our appliance, we have 2 variants: one with an HD44780 display and
> some machines without. I'd like to have some mechanism in lcd4linux,
> which makes the daemon exit gracefully when there is no display on the
> paralell port.
> 
> Does lcd4linux offer something like that?

Well, as usual, that depends.

If you don't use busy-flag checking, the communication is pure
unidirectional, and there's no way to detect wether there's a display
connected or not.

If you use the busy-flag, there's a possibility to read back at least
some status information from the display. LCD4Linux uses this to test
wether the display can accept a new command or not.

At the moment there's kind of a fallback, which means that if the
busy-flag check fails several times, the HD44780 driver goes back into
normal unidirectional mode (using some worst-case delay values).

But I'm afraid this check won't be sufficient: normally the display is
that fast (or the parport is that slow) that the first 'check-busy' test
reports the display as idle. And a successful busy check is done by
testing some signals for logical 'high' (or 1), which is the default
state if the pin is not connected (there's sort of a 'pull up resistor'
built into the parallel port).

I'll have to dig into the HD44780 specs to find out if there's another
possibility to check for a connected display. We could implement such a
test into the code (if there is one).


bye, Michael

-- 
Michael Reinelt <re...@eu...>
http://home.pages.at/reinelt
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