> From: "Larry P. Thomas WA0GWA" > I'm working on some PIC devices and intend to use some I2C code to code up > the slave routines. I found routines to let me use PC parallel port as > master to test my slave devices but the references I found all use some sort > of hardware on the PC port such as a 74LS05 to aid in the bidirectional I2C > implimentation. Since some of the parallax products seem to use I2C to talk > between the PC and the target system without any extra hardware at the PC > end, I was wondering if anybody had any info on how to do this. The > hardware is simple enough, I just was trying to avoid a trip to the city to > get the parts for the I2C PC port interface. Since I2C requires open collector drivers, you may be able to get away with using a diode (cathode to the output) to simulate one. This is a bit bodgy since the diode drop (0.6V) may put you over the guaranteed logic low level. Try dusting off your old OA91 germanium diodes (0.4V) or dismantle an old SMPS for its Schottky diodes (0.4V or less). The 100 Amp stud-mount types are best - throw away the heatsinks though 'cause you won't be a-needin'em. Otherwise, use a couple of small NPN's (2n2222) in common emitter/open collector mode - use resistor in base drive and invert the output levels from 'standard' I2C, because the transistors will act as inverters. The only other components required are 2 x 4k7 pullup resistors (and obviously connections to two input lines on the printer port). Regards, SJH Canberra, Australia