Parallel port/i2c interfacing question
 

I have an i2c interface built around the old Philips design of a 74LS05 and 10k pull-up resistors on my PCs parallel port. (If you don’t know this design see http://www.raphnet.net/electronique/...ts/lpt-i2c.txt)

It works OK(ish) with the standard Linux bit-bashing driver, but every now and again, say once every 100 I/O operations it fails. Until now, I’ve only had one i2c device on the bus and I’ve programmed around the problems by doing a re-read or re-write if my original I/O request failed.

I now want to add another device, and when I do, nothing works at all. The waveforms get very distorted. Either device on it’s own is OK, but together, no joy.

I am wondering if there is an inherent fault in this design of interface and if the pull-up values need changing if I add more devices. My i2c bus is less than 30cm long.

Has anyone here experience of using the Linux drivers to interface to external hardware ? If so, do you use the Philips design/driver, and if not, what ?

Is there an i2c guru reading this that can give me some advice about bus loading ?