Interfacing the Standard Parallel Port http://www.senet.com.au/~cpeacock
Interfacing the Standard Parallel Port Page 7
Base + 1 Status Port Read Only Bit 7 Busy
Bit 6 Ack
Bit 5 Paper Out
Bit 4 Select In
Bit 3 Error
Bit 2 IRQ (Not)
Bit 1 Reserved
Bit 0 Reserved
Table 5 Status Port
The Status Port (base address + 1) is a read only port. Any data written to this port will be
ignored. The Status Port is made up of 5 input lines (Pins 10,11,12,13 & 15), a IRQ status register and
two reserved bits. Please note that Bit 7 (Busy) is a active low input. E.g. If bit 7 happens to show a
logic 0, this means that there is +5v at pin 11. Likewise with Bit 2. (nIRQ) If this bit shows a '1' then
an interrupt has not occurred.
Base + 2 Control Read/Write Bit 7 Unused
Port Bit 6 Unused
Bit 5 Enable bi-directional Port
Bit 4 Enable IRQ Via Ack Line
Bit 3 Select Printer
Bit 2 Initialize Printer (Reset)
Bit 1 Auto Linefeed
Bit 0 Strobe
Table 6 Control Port
The Control Port (base address + 2) was intended as a write only port. When a printer is
attached to the Parallel Port, four "controls" are used. These are Strobe, Auto Linefeed, Initialize and
Select Printer, all of which are inverted except Initialize.
The printer would not send a signal to initialize the computer, nor would it tell the computer to
use auto linefeed. However these four outputs can also be used for inputs. If the computer has placed a
pin high (e.g. +5v) and your device wanted to take it low, you would effectively short out the port,
causing a conflict on that pin. Therefore these lines are "open collector" outputs (or open drain for
CMOS devices). This means that it has two states. A low state (0v) and a high impedance state (open
circuit).
Normally the Printer Card will have internal pull-up resistors, but as you would expect, not all
will. Some may just have open collector outputs, while others may even have normal totem pole
outputs. In order to make your device work correctly on as many Printer Ports as possible, you can use
an external resistor as well. Should you already have an internal resistor, then it will act in Parallel
with it, or if you have Totem pole outputs, the resistor will act as a load.
(96 pagine)
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