PARALLEL INPUT/OUTPUT INTERFACE
The general-purpose bi-directional parallel interface is a 25-pin connector on the back
panel of the computer. This connector is generally used for a parallel printer.
For each data byte written to the parallel port register, the hardware automatically
generates a pulse on the data ready pin. The acknowledge pulse from the parallel device
is hooked up to an interrupt. For pin connections and timing, see Appendix E and F.
SERIAL INTERFACE
A 25-pin connector on the back panel of the computer serves as the general purpose
serial interface. This connector can drive a wide range of different peripherals, including
an external modem or a serial printer.
For pin connections, see Appendix E.
INTRODUCTION TO SERIAL CIRCUITRY
The Paula custom chip contains a Universal Asynchronous Receiver/Transmitter, or UART.
This UART is programmable for any rate from 110 to over 1,000,000 bits per second. It
can receive or send data with a programmable length of eight or nine bits.
The UART implementation provides a high degree of software control. The UART is capable
of detecting overrun errors, which occur when some other system sends in data faster
than you remove it from the data-receive register. There are also status bits and
interrupts for the conditions of receive buffer full and transmit buffer empty. An additional
status bit is provided that indicates "all bits have been shifted out". All of these topics are
discussed below.
SETTING THE BAUD RATE
The rate of transmission (the baud rate) is controlled by the contents of the register
named SERPER. Bits 14-0 of SERPER are the baud-rate divider bits.
- 250 Interface Hardware -
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