After having dealt a little with systems based on the 6502 and the Z80, it is now time to have a closer look into the predecessor of the IBM PC, the Intel 8085.

Craig Andrews introduced a very interesting and advanced concept, the SBC-85. Craig designed his layout freely and encourages everyone to build the system themselves based on their published data.

His project website can be found here.

The project is very well documented. A comprehensive user guide is available for each board, which describes the board and its technical concept in detail. BOMs with detailed information, circuit diagrams and of course the Gerber designs data are also available.

Using the Gerber files, you can have the circuit boards manufactured yourself at the usual PCB houses (e.g. in China) at fair prices.

Unfortunately, the site is extremely difficult to use - all links only work if you use "open in new tab".

Therefore please use the direct links to the respective sub-pages used here.

The following PCB I have recently ordered at  JLCPCB in China:

• Backplane-4 v1.0 
• SBC-85 CPU 2.x 
• Memory Expansion v1.0a 
• Cassette Tape Interface v1.0 
• Bus Monitor Supreme v1.2b 
• SPIO v1.0 (Serial-Parallel I/O) 
• Bus Protoboard v1.0d 
• Port I/O ProtoBoard v0.9 

The complete overview of boards can be found on Craigs website, under Dokumentation.

The heart of the system is of course the CPU board, which was released in two versions. I have used V2.x straight away, i.e. the SBC-85 CPU 2.x

The User Guide can be found here, the Gerber files are here

The SBC-85 is an 8085 based single board computer, which can work completely stand-alone (without the other circuit boards of the system). In addition to the CPU, the board contains: 64k RAM and ROM (or EPROM), a serial RS232 port and an 8155 module with 1k RAM, I/O and timer.

The system can be expanded by a 120-pin expansion bus, using standard PCI slots. For this purpose, there is a separate backplane board existing, containing 4 slots, the module Backplane-4 v1.0

As usual, a detailed User Guide is existing, as well as the Gerber files to create the PCB.

A regulated 5V power supply (plug-in power supply with 2.1mm barrel connector) is used. In stand-alone operation, the CPU board is connected directly there; when operated via the backplane, the power supply unit is of course connected there, and the cards are supplied via the bus.

There is also a memory expansion existing, the board Memory Expansion v1.0a

User Guide and Gerber Files are accessible, as usual.

The board has four memory slots that can be equipped with any combination of EPROMs of type 2732, 2764, 27128, 27256, or with RAM of type 6264. That makes up to 128KB ROM, or 32KB RAM, or anything in between.

The SPIO v1.0 (Serial-Parallel I/O) board, which offers a serial and a parallel interface, serves to supplement additional interfaces.

User Guide and Gerber Files are there.

The I/O board offers an RS-232 serial port with baud rates of up to 19.2K using an Intel 8251 USART, as well as 24-bit parallel input/output pins using an Intel 8255 Programmable Peripheral Interface (PPI). There is also an 8-bit I/O port with onboard LEDs.

A cassette recorder can be connected as a “mass storage device” via the Cassette Tape Interface v1.0.

User Guide and Gerber Files.

The cassette recorder is connected via two 3.5mm audio (jack) sockets. The Intel format is used for recording - details can be found in the above user manual.

The highlight of the system is the Bus Monitor Supreme v1.2b board.

User Guide and Gerber Files

This board offers a representation of the 16-bit address and the 8-bit data signals of the system bus via six 7-segment displays.

Both are also displayed (in addition to other status displays) in binary form via corresponding LEDs. Furthermore, the program sequence of the CPU card can be controlled via the board: single-step, slow-step at different speeds (from approx. 0.5 Hz to 60 Hz) are possible. Addresses can be loaded and, for example, breakpoints can be set.

There are also two boards available for building your own circuits.

The Bus Protoboard v1.0d  is designed as a plug-in card for the system bus and offers easy access to the signals of the bus and an area for building your own, small circuits (Gerber Files).

Using the Port I/O ProtoBoard v0.9, you can also build your own, small circuits that can access the I/O ports (Gerber Files).

There are a few more expansion cards, but I haven't dealt with them any further - please refer to the project website.

That much for an overview. In further Bog posts I will go into more details on the structure of the individual boards.