logajan@ns.network.com (John Logajan) (08/23/89)
Someone wrote to me about my DeskJet Plus single transistor fix to the power-up-sequence problem and STROBE line drive buffering. They were still concerned that the data bits need buffering -- I hope to show with my measurements that there is nothing to worry about. The data lines do not need to be buffered. I measured the resistance of the DJ+ data pins. This was done by connecting a potentiometer between a data pin and ground. The pot was then adjusted until it read 2.5 volts, or 1/2 the applied no-load voltage. 2.5 volts occured when the pot was set to 2000 ohms. So it is safe to say that the DJ+ has at least 2000 ohm pullup. I further measured at .8 volts and .4 volts, which had 300 and 100 ohms respectively. You can see that the current draw increases non-linearly as the voltage drops because you are not only drawing current from the pull-up, but also from the TTL input. I also re-measured the actual voltage present when the Atari was pulling a data bit to zero. That was between .2 and .3 volts, let's say .3. Actually, let's say .4, because I don't have a current measurement for .3. .4v/100ohm=.004amp .004amp*.4volt=.0016mw Now the Atari (according to a quote from a manual) is spec'd to be able to drive 3000 ohm loads on the data pins. They would have to spec it at the worst case maximum low voltage level, the TTL standard being 0.8 volts. If 0.8 volts is at the driver, that leaves 4.2 across the pullup resistor/ TTL input. 4.2volts/3000ohms=.0014amps 1.6 ma is considered a standard TTL input low load. So it looks like Atari is claiming the data pins can drive one TTL load. Power wise, 0.8volts*.0014amps = 0.00112 watts, or 1.12 milliwatts. So we have the difference between 1.6 and 1.12 milliwatts, the case would even be better if I had the current measurements for the "real" .2 to .3 volt logic zero level. The power difference at this level is miniscule and not worth worrying about. These drivers are designed to withstand direct shorts to the power rails -- the difference here is piddly in comparison. Thus I'd say the Atari is quite safe running the DJ+ without buffers (on everything except the STROBE line, which needs buffering for other reasons also.) So, I think my one transistor buffering is a complete solution to the power-up problem and STROBE loading. -- - John M. Logajan @ Network Systems; 7600 Boone Ave; Brooklyn Park, MN 55428 - - logajan@ns.network.com / ...rutgers!umn-cs!ns!logajan / john@logajan.mn.org -