Dick Smith Super-80 Computer-Technical Description-pcb

- Feb 22, 2017-

Dick Smith Super-80 Computer-Technical


Technical description

Super-80 computer logic board (reverse side)

The Super-80 was based on the Zilog Z80 8-bit microprocessor. As standard, it had 16 kB of dynamic RAM in the form of eight 4116 RAM chips. RAM could be expanded to 32 kB or 48 kB through the addition of rows of eight 4116 RAM chips.

The computer was assembled on a single double-sided printed circuit board. The board was supplied in a light cardboard sleeve that appeared to be an LP record sleeve, having the words "Dick Smith Super 80 Microcomputer Kit Printed Circuit Board" and the part number "Cat H-8402" printed along the spine.

To keep the price of the computer and the component count down, a novel technique was used to implement the video display. Instead of an expensive video display controller chip with dedicated memory, the Super-80 used discrete TTL logic to implement the video display and 512 bytes of system RAM was shared between the video display and the CPU. Fifty times per second, the CPU was turned off for around 10 ms by asserting the Z80 BUSREQ (DMA) pin. The video display circuitry would then read from the shared RAM while it refreshed the image on the screen. In addition to a 50% degradation in processor performance, this meant that it was not possible to perform any accurate timing in software, since the programmer had no control over when the next video display refresh cycle would occur. The video display could be switched off under software control for greater processing speed, or when accurate software timing was required. The most common situation in which that occurred was when the built-in cassette interface was being used. The location of the 512 bytes of video memory was normally at the top of the available RAM, but could be changed by writing to an I/O port.

The keyboard was part of the main computer PCB, but before assembly, the constructor could opt to cut the keyboard section of the printed circuit board off and connect it to the main board with ribbon cable. The keyboard was wired as an 8 × 8 matrix and connected to the computer via the two 8-bit ports of a Z80 PIO chip. Pressing the keys <CTRL>, <C> and <4> at the same time generated an interrupt that would perform a "warm start" of the monitor program. The keyboard 'read' routine supplied in ROM was "negative edge triggered" and would block while a key was down. As a result, most action games incorporated their own keyboard driver.

The standard computer had no serial or parallel I/O as such, relying on the optional S-100 bus interface for I/O and expansion. A 10-pin connector at the back of the board was labeled "PORT" and had power, as well as a pair of digital outputs and two available digital input lines. The connector was for a future RS-232 / 20 mA current loop serial interface,[3] but that was never implemented.

Mass storage was available in the form of a cassette tape interface running at 300 baud. Accessing the cassette interface required the video display to be switched off, so an LED was provided to show activity during a tape load or save operation. The LED would change state each time a 256-byte block of data was successfully transferred.

The Z80 interrupt line was connected to the keyboard PIO and the "Non Maskable Interrupt" line was not connected.

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