Small Computer 101

A prototype Small Computer has been built and works as expected.

It has a Z80 processor, 128k bytes of FLASH and 128k bytes of RAM. A simple paging system allows access to all of the memory. A serial port is included for connection to a terminal. Expansion is available through an RC2014 bus connector.

This is a slightly simplified version of paper design SC002.

SC101-image

Component count:
5 x Integrated circuits (Z80, 128k byte RAM, 128k byte FLASH, 74HCT259, 74HCT4075)
1 x Oscillator
Assorted passive components

SC101-schematic

Schematic (PDF)

Circuit Description (PDF)

Parts and Costs (PDF)

The user interface is provided by a terminal connected via the serial interface. Typically an FTDI cable will be used to connect the two.

The ROM can hold the Small Computer Monitor software, plus anything else required.

Paging allows the ROM to be removed from the memory map, under software control, and replaced by RAM. Two 64k byte RAM banks are available which can be selected under software control.

Outputs are provided by an 8-bit addressable latch. Address line A0 provides the ‘data’ bit for the latch. This mechanism has a number of advantages over the more common 8-bit D-type flip-flop arrangement. Firstly, when only one output bit is to be controlled there is no need for the usual bit manipulation and storing a copy of the output byte in RAM. Secondly, when setting or resetting a bit no registers or flags need to be modified as the single instruction “OUT (n), A” can be used (where the value of ‘A’ is irrelevant).

There is only one input and this is achieved by feeding the signal to the data bus via a resistor. If no other device is driving the data bus, the bus logic level is determined by the input. This is a bit of a dirty trick and has the disadvantage of loading the bus, but it is simple and cheap. You can only use the trick once on each data bus signal.

 

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