"Interfacing the IBM PC parallel printer port" - читать интересную книгу автора (Stewart Z.)

a parallel port from Taiwan that tristated all the data lines between
every write to the data lines. This presented a pattern of FF to the dongle,
but had no effect on the printer since there was no strobe. In this case
the software was writing a sequence of bytes to the dongle and it didn't
work due to the FF in between each expected real byte."

"The dongle also draws power from the signal lines--a definite no-no.
Dongles should be designed to operate all the way down to the minimum TTL
low or at least to minimum Vout High of 2.4V. If your dongle doesn't work
but your printer works fine then it is almost certainly the fault of the
dongle--not the parallel port. Software with an incompatible dongle to the
parallel port on a machine will not be usable on that machine--one more
reason to not penalize the legitimate buyers of software."

14. Other devices:

From: [email protected] (George Pontis)

"Another area that might be of interest in your document would be some
comment on the parallel port extenders. I have a xmit/rcv pair from LinkSys
that I bought from Fry's Electronics for about $70. They convert the parallel
signal to a serial data stream, using the signal and control lines for power.
My set was working fine until I added a hardware dongle for an expensive
Windows application. Then, printing ceased to work reliably. I took the
transmitter apart and partially traced the schematic. They have used 7
diodes to suck power from pins 13, 14, 15, 17, 1, 2, and 3. Also, they
connected pins 15 (ERR) to 16 (INIT). The strobe line is coupled in to a
flip-flop, which starts clocking the parallel loaded data."

Comment from Zhahai - with both a dongle and a parallel port extender trying
to draw power from the port's data or control (not power) lines, it's not
surprising if things don't always work! It must be really pushing the specs.

15. Transferring Data Via Ports

There are three basic ways to wire IBM printer ports together to transfer
data between them. The most common is to wire D0 - D4 (or D3-D7) from one
port to S3 - S7 from the other port, and vice versa. In this way, 4 bits of
data can be put out by one (eg: D0 - D3) and read by the other (eg: S3 - S6);
the other bit (eg: D4/S7) can be used for synchronizing (eg: Data Ready).
If data is transferring one way, the Dn/Sn path the other direction can be
used for acknowledgements. In this scheme, you must remember that S3 - S6
read the inverse of the other computer's D0 - D3 data bits, but S7 has the
same value as D4 (or D3 - D6 and D7 respectively, for the alternate wiring).
Also remember that S6 and S7's mapping to pins is swapped in order from the
others.

16. Transfer Modes and Cables

Mode 1A: nibble mode, using Data Out to Status In connection