LIGHT PEN

The light pen input latches the current screen position into a pair of registers (LPX, LPY) on a low-going edge. The X position register 19 ($13) will contain the 8 MSB of the X position at the time of transition. Since the X position is defined by a 512-state counter (9 bits), resolution to 2 horizontal dots is provided. Similarly, the Y position is latched in its register 20 ($14), but here 8 bits provide single raster resolution within the visible display. The light pen latch may be triggered only once per frame, and subsequent triggers within the same frame will have no effect. Therefore, you must take several samples before turning the pen to the screen (3 or more samples average), depending upon the characteristics of your light pen.

RS-232 INTERFACE DESCRIPTION

GENERAL OUTLINE

The Commodore 64 has a built-in RS-232 interface for connection to any RS-232 modem, printer, or other device. To connect a device to the Commodore 64, all you need is a cable and a little bit of programming.

RS-232 on the Commodore 64 is set-up in the standard RS-232 format, but the voltages are TTL levels (0 to 5V) rather than the normal RS-232 -12 to 12 volt range. The cable between the Commodore 64 and the RS-232 device should take care of the necessary voltage conversions. The Commodore RS-232 interface cartridge handles this properly.

The RS-232 interface software can be accessed from BASIC or from the KERNAL for machine language programming.

RS-232 on the BASIC level uses the normal BASIC commands: OPEN, CLOSE, CMD, INPUT#, GET#, PRINT#, and the reserved variable ST. INPUT# and GET# fetch data from the receiving buffer, while PRINT# and CMD place data into the transmitting buffer. The use of these commands (and examples) will be described in more detail later in this chapter.

The RS-232 KERNAL byte and bit level handlers run under the control of the 6526 CIA #2 device timers and interrupts. The 6526 chip generates