On Thu, 19 Sep 2002, James Williams wrote:

*>Why don't you use the nibble read to read the data?  Or you could even
*>do a byte read.  It is strange to find people still using the status
*>pins to read data from a device.  Especially when you can do byte read
*>and nibble reads without sacrificing no less that 4 input pins.
*>
*>Also, a better interface with multiplexing is a solution around the 13
*>pin limit.  I have not seen this as a problem.

Each project has its own requirements. One of them used a 688 comparator
to 'guess' a 8 bit input value on a port that could not read back. The
output of the 688 was wired to the paper empty status pin. This was a hack
but it works (slowly). Another version of the same device used a 257
multiplexer to read nybbles as you suggest. The nybbles were read through
four of the status lines, not the data lines.

The real problem is, that I often write the drivers myself for other OSes,
some just embedded kernels, make everything work fine, then someone comes
and says let's use this with a standard PC (by this he means
compatibility with the 1000 different kinds of PC parallel ports out
there). What they usually have in common (or had until recently):

- Made in China
- No specs
- Works with Windows or original BIOS and who knows what's inside
- Strange register set quirks that make programming 'fun'
- Sometimes missing features (like no readback from the data port).

Newer ports do not have this kind of problems.

Peter

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