elee24@castle.ed.ac.uk (H Bruce) (07/30/90)
I realise that there has just been a discussion on the net about reading from the parallel port, however at that time I had no interest in the subject - but things have changed ! So is this possible and if so how ? Thanks, Henry Bruce.
grege@gold.GVG.TEK.COM (Greg Ebert) (07/31/90)
In article <5399@castle.ed.ac.uk> elee24@castle.ed.ac.uk (H Bruce) writes: >I realise that there has just been a discussion on the net about reading from >the parallel port, however at that time I had no interest in the subject - but >things have changed ! > The 'standard' parallel port has read-back capability, but it is INTENDED only for reading-back what was last written. This is exploited in auto-sense software which pokes around to see 'what is where'. Anyway, when you attempt to read-in via the parallel port, you have to overdrive or disable the output circuitry. Some manufacturers might have termination networks which have resistors in series with the output driver, and input the exact level of the connector pin. In this instance, you could shove a '1' into the port, write a '0', and still read-back a '1' because the resistor helps to isolate the driver from whatever you're reading. If your port isn't wired this way, you need to disable the output driver itself. Usually, its a 74XX374 (maybe a 74XX373). If you yank pin 1 high, the output driver is disabled. Maybe a toggle switch is a good idea. I would leave the driver enabled (pin 1 = low) while BIOS is doing it's thing, otherwise it might not get installed [dump 40:0 and look for 02 78 or 03 78]. The PS/2, (I'm told) has true readback capability. There is a spare pin on the keyboard controller which enables or disables the driver. # # # # # # # # # # # BONUS QUESTION : Anyone know why Novell non-dedicated runs slower on a 386-33 (worse yet on a 486) than on a 4.77Mhz XT ? I'll post the answer next week.