
Description of PC-LPT/IEEE488 2.2
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This is the attempt of using a PC printer port to build an IEEE488
interface to be used with the Commodore PET and business line of computers.

Data lines and reading the control lines is simple and should be 
self-explanatory.

The CBM use special timeouts in their IEEE488 routines.
The PC should not be worried with this as it has 
enough other things to do. So this part of the handshaking
used to be done in hardware.
However, the hardware handshake gave rise to various problems,
it did not work. Therefore now all handshake lines are set and
checked by the computer.

Basically IC1 is used as an address decoder. The LPT data lines 
are used to transfer data to the databus (IC2) and control (IC5)
latches.
By selecting one of the corresponding nibble outputs of IC4
(databus) or IC6 (control) the bus lines can be read.

By setting the Tx bit and clearing the DAV bit in the control
register the databus is put in a loopback mode, i.e. the data 
input lines show the data that is put into the databus output
latch. This used for testing if the interface is there and
for measuring the speed of the interface.

