The Parallel Port: Introduction
Some signals are electrically inverted, i.e., a logical 'true' is
indicated by low level on the physical line/pin.
It may be confusing that some line states are sometimes inverted on
the PC's port registers. This is totally independent from the electrical
inversion. This whole document will only talk about physical voltage
levels, so the port inversion is only important when programming kernel
drivers and the like.
The electrical inversion is indicated in the signal name by an
initial slash, e.g. /Init.
The (maybe addititional) PC port inversion is shown in the
following table.
The pin number given in the table refers to the sub-D 25 pin connector.
| Name |
Pin# |
Direction |
Port Logic |
Description |
| /Init |
16 |
H to P |
Positive |
Low level signals
a reset condition regardless of the current state the
device is in. My advice is
that the peripheral sets all lines to Z-state, although this is
not necessary.
|
| /Select |
17 |
H to P |
Inverting |
Low level signals that
the parport is in Centronics mode. In 1284 modes,
the signal is inactive (i.e., high).
|
| D0,...,D7 |
2,...,9 |
both |
Positive |
The data lines are H to P in Centronics and
nibble modes, and bi-directional in other 1284 modes.
If unidirectional, they will be shown in host
color: D0,...,D7.
|
| /Strobe |
1 |
H to P |
Inverting |
High-low edge
signals that there is a byte of data on the data lines
D0,...,D7.
|
| /AutoFeed |
14 |
H to P |
Inverting |
Shows whether the printer must introduce a
automatic line feed (ASCII 10 / 0x0a) when a carriage
return (ASCII 13 / 0x0d) is found in text mode.
|
| Select |
13 |
P to H |
Positive |
Used by the peripheral to indicate that
it is on-line (=selected).
|
| Busy |
11 |
P to H |
Inverting |
Used by the peripheral to stop
the host sending more data.
|
| /Ack |
10 |
P to H |
Positive |
Uses by the peripheral to acknowledge
a data transmission.
|
|
12 |
P to H |
Positive |
By this the peripheral shows that
there is no more paper in the printer.
|
| /Error |
15 |
P to H |
Positive |
By this the peripheral shows that
there is some problem.
|
Example
When a PC wants to indicate a reset condition, it has to set the
/Init line to low level, because
it is electrically inverted (as shown in the name). Because the PC
has positive logic on that pin, it has to write a 0 into the
corresponding bit in the register.
Note
The host-to-peripheral signal /Select is sometimes
called /Select_in to clearly distiguish it from
the peripheral-to-host signal Select.
In contrast to normal convention, signal timing diagrams will
be shown in top-down direction instead of left-to-right. This has
the advantage that explanatory text can easily be provided for
each step.
|
|
|
= |
Driven high |
|
|
|
= |
Driven either high or low |
|
|
|
= |
Driven low |
|
|
= |
Insignificant (probably driven, but maybe not) |
|
|
|
= |
Not Driven, Z-State |
