On Tue, 15 Jul 2014 11:53:22 +0100, "Arfa Daily"
wrote:

I'm assuming that the 'chip' on each cartridge is a simple dedicated
microcontroller, so some level of Vcc would be required to run that, and it
would make sense to design the chip to run off a rail that would provide
suitable levels for the bus signals as well

Yep. That's what I just found. HP C7250 AIO printer.

There are 4 pins on each cart. I'll arbitrarily number them 1-4 from
top to bottom.
1 +3.3v DC
2 Data cartridge -- printer
3 Data cartridge -- printer
4 Ground
I verified that the +3.3v DC and ground were there full time when the
printer was powered on. The ground pin shows DC continuity to the
shield on a nearby USB jack, which was convenient for grounding the
scope probe. I saw short bursts of data during power up and during
power down. After the data burst on pin 3 (data into the cart), where
was a very short burst on pin 2 from the cartridge back to the
printer. I couldn't capture the bursts because my ancient Tek 422
scope doesn't have storage. That's certainly NOT I2C.

I didn't want to try printing with all the scope probes and
connections attached.

Offhand, the system looks incredibly crude. No clock, no time sync,
no rolling code, no real security. Probably just some bits to
identify the type and color of the cart, and a challenge/response
system to identify that it's a genuine HP car. Oh yes, the date of
manufacture so that HP can declare the carts useless because the ink
has "expired". It wouldn't take much to clone the chips.

However, I have a better idea. It wouldn't be too difficult to rip
the chips off the carts, run 4 wires to a PIC controller, and give the
printer whatever response it needs to keep it happy.

And I, in turn, leave this to Jeff L who has these things piled high
(literally !) at his place ... :-)

Worse than that. One of my customers called about 2 weeks ago with a
problem. He's recovering from surgery and needs two bedrooms full of
eJunk cleared out so the daughter in law and the grand brats can move
in and help him out while he recovers. I now have a museum of antique
eJunk, most of which is in immaculate condition. Another customer
moved offices, and used my office doorstep as their recycling and
eWaste depository. I sacrificed one workbench and one desk, turning
both into vertical storage, almost to the ceiling. Hopefully, the
fire marshall will not come visiting.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558