- Video Link: Z80 Retro #2 - Block Diagram
The CPU Board is built around the Zilog 8 bit Z80 CPU. It has a 16 bit wide address bus which means it can address a maximum of 64kB of memory.
John, the designer, has tried as much as possible to re-create the original Single Board Computer hobby system that was so popular in the early eighties but with a modern twist. It is built almost entirely of modern components. The Z80 Processors are still in production and all the logic ICs are still being made. The only components that are either new old stock or second hand stock are the VDP and the VDP Memory. With the VDP being an optional add-on, you could get away with thinking that John has absolutely achieved this goal.
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CPU
The CPU Board is built around the Zilog 8 bit Z80 CPU. It has a 16 bit wide address bus which means it can address a maximum of 64kB of memory. It also has a separate I/O Bus used to access peripherals.
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RAM
512kB SRAM divided into 16 banks of 32kB each.
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FLASH
128kB FLASH ROM. The CPU boots from the flash ROM, which in turn is responsible for loading the first 32 blocks of storage from the SD Card into the TOP (Bank 15) bank of RAM and then jumping to the first address in that bank.
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CTC
This a general purpose counter timer. It has a selection of programmable timers that are used to drive the baud rates for serial communication via the CON and AUX RS232 ports.
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SIO
A dial serial input output device which is connected to the RS232 chip.
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RS232
Responsible for converting the TTL (5V) logic signals into the +12V / -12V required for a standard RS232 connection.
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I/O Address Decoder
These ICs convert requested I/O Addresses into chip enable signlals for the input buffer and the two D-Type latch devices.
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GP I/O and Printer Out
These chips are responsible for interfacing to the SD Card, Printer as well as holding the memory bank latch.
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SD CARD
Block storage device interfaced over bit-banged SPI via the GPIO system.
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Power
Power is provided through a USB Type B Connector on the board. This connector only provides Power. There is also an onboard voltage regulator that is used to provide 3.3V to the SD Card adapter.
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System Clock
A 10MHz Full Can Oscilator provides the 10MHz signal to the Z80 CPU.
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Baud Rate Clock
Optionally divided by the CTC to allow for slower baud rates on the SIO. By default, the Z80-Retro runs serial I/O at 115200kbps.
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CPU Connector
This connector is a 40pin header that can be used to interface with the two daughter boards. It provides all the same signals as those on the CPU.
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Reset Circuit
A momentary push button style switch that brings the RESET signal low while held down. Used to COLD BOOT the CPU, reset the FLASH latch, and start everything up from scratch.
The Z80-Retro has 3 primary interfaces for communication with external devices.
- RS232 CON The main serial interface used to connect to the Z80-Retro over an RS232 serial connection. This is the default serial port to connect to your host PC via an IDC to DB9 adapter cable.
- RS232 AUX Used for auxillary serial communication. The Baud rate of the AUX port can be programatically controlled to support slower rates for older devices. Typically, one would use this to connect to another serial device or modem.
- Printer A 26pin IDC connector requiring an IDC to DB25 adapter cable to connecting to a line printer.
There are two daughter boards available for the Z80-Retro.
This optional programmer is designed to interface directly to the Z80-Retro CPU Board and a standard Raspberry Pi Header. When this is connected and you run the required tools, it will take direct control of the address and data buses by asserting the BUSREQ signal to the CPU. Once it has control of the buses, it will program the 128kB FLASH ROM chip.
It is also possible to program the FLASH ROM directly using another kind of programmer that supports the specific FLASH ROM device.
The programmer has on board voltage level shifting to protect the 3.3V pins on the Raspberry Pi.
A graphics card for the Retro outputting a composite NTSC signal. It has its own video memory and can be controlled by the CPU over the I/O Bus.
The Z80-Retro is designed to work with CP/M version 2.2. There are a range of software options for CP/M 2.2 that can be loaded on to the SD Card and will work.
CP/M is designed as a serial only operating system and works very well on the Z80 platform. It is, therefore, the natural choice of OS.