Commodore Amiga A500 Technical Reference Manual page 108

Digital Video
LIGHT PEN (/LPEN)
PORT CONNECTIONS
8 Bit Parallel Port
(PD0-PD7)
Parallel Port Handshake
(/ACK)
Other Port Lines (BUSY,
POUT, SEL)
The remaining 8 bits of digital video are available on this connector.
The signals are Red 0-2 {pins 2. 3. 4). Green 0-2 (pins 6. 7, 8), and
Blue 1 -2 (pins 10 and 11). The timing of the digital video is not
tightly specified. Developers wishing to use this should contact Com-
modore for further details.
This is an input to the Agnus light pen input. This signal should go
low in response to the lighting of a pixel on a video display monitor.
The Agnus chip latches the raster position that was in effect when
the /LPEN signal goes low, so an application can follow the position
of a light pen on the screen. Pin 19.
Most of the signals from the bidirectional parallel port (printer port)
are available on this connector as well, along with a few others.
The 8 bit bidirectional parallel port most commonly used to drive a
Centronics interface printer externally is accessable here. It can be
used to control various aspects of a complex video interface device.
The port lines PD0-PD7 are on pins 23 to 30 of this connector.
This is the acknowledge (/ACK) input, the same as the acknowledge
input to the parallel port. Driving this with an output from a Video
Card can cause a level 2 interrupt to occur through the 8520 CIA
device this is connected to, based on the programming of an 8520
register. On pin 20.
Connector pins 18 (BUSY) and 16 (POUT) are general purpose I/O
signals that together can also function as a synchronous serial data
port driven by an 8520 CIA device. In normal printer use. the BUSY
signal is used to indicate printer buffer full to the Amiga, while
POUT is used to indicate the printer paper is out. For serial port
usage, BUSY is the serial clock, POUT is the serial data line. These
should be driven with open collector devices if the Video Card uses
them as inputs to the 8520. The SEL signal, on pin 21, is a general
purpose I/O port, usually used as a device select signal on the parallel
port.
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