Table of Contents
GRC, a new ecosystem for 8-bit processors of 70's and 80's
Introduction
Generic Retro Computer (GRC) is a new concept in modular system for 8-bit (and some 16-bit) processors of 1970's and 1980's. The processor can be Z80, 8085, 6809, 6502, 68008, and other 5-volt, 8-bit processors or 16-bit processors with 8-bit bus. The processor shares the same hardware resources of CPLD, RAM, and CF disk. Other boards such as video display, sound, and discrete I/O can be added to the core GRC system.
Design Concept
Backplane
GRC is a modular computer system consists of an expandable backplane, 100mm X 100mm, with six female 2×25 connectors and a dedicated slot for disk-on-module. Each board is 100mm X 40mm with a corresponding 2×25 male connector that plugs into the backplane. Table 1 shows the signal assignments of the backplane connectors.
Processor Board
Processor board is a 5Volt 8-bit processor with most of its signals connected to the 2×25 connector. The addresses, data, power, reset, clock signals are fixed, but the processor control signals vary depending on the processor in use. The programmable CPLD will accommodate the different processor control signals and drive the rest of the system appropriately.
8085 board
6809 board
68008 board
32008 board
CPLD Board
CPLD board is the heart of the GRC. A minimum PRCC is consists of a CPLD board and a processor board. CPLD contains a simple bootstrap ROM, an internal serial port, and necessary decoding logic to interface to the specific processor and control the RAM and compact flash drive. The CPLD serves as the diagnostic module when bringing up a GRC system initially.
RAM/Flash Board
RAM/Flash board is the only memory in the system. It consists of a 512K flash memory and a 512K RAM memory. The flash can be reconfigured as a 512K RAM making it a full megabyte board. Both RAM and flash are divided into multiple 32KB bank for most 8-bit processors. There are 4 bank registers on the CPLD that can be controlled under software. For processors with larger than 64K memory space such as Z180, Z280, and 68008, processors have direct control of the RAM's address lines.
Future Boards
Future modules may be multi-channel serial port, video display, sound card, discrete I/O, SPI/I2C/parallel ports and others.