Memory Card: Revision B

Last Updated: November 09, 2005 09:48:34 PM

Not Yet Produced

Revision B not yet produced of the memory card with the RAM installed at 0x0000 – 0x7FFF and the EPROM at 0x8000 – 0xFFFF. ( FIX test with new PCB and picture) The jumper blocks on the left enable and disable 4K memory blocks. This card has RAM 0x0000 – 0x6FFF and EPROM 0x8000 – 0xEFFF areas are enabled.


The memory card contains 64K of RAM and 64K of EPROM made up four 32K x 8 chips (2 RAM, 2 EPROM). The RAM and EPROM are each spread across the entire 64K address space of the 1802. (FIX ADD JUMPER DESCRIPTION). The card was designed with this flexibility to accommodate old software of my own and new software such as ELF/OS. The card replaces the original System-4 16K Ram card.

Issues/todo log

Date Description Status

Schematics, board layout, and operational description

Revision Production Date Schematic Layout Operational Description
A Dec-16-04 mem3.sch mem3.brd Rev. A ops notes
B Current Development Board


mem3-b.sch mem3-b.brd Rev. B ops notes

System Bus

Last Updated: October 31, 2005 11:13:27 AM

The bus is about as simple as can be. It is made of Vero board which is comprised of parallel strips of copper on a .1 inch grid. The card edge holders are optional. See the additional bus pictures for other views of the bus and connectors.


Not much to say here, look at the Bus Definition Rev. B and additional bus pictures for design details.

Issues/todo log

Date Description Status
Dec-2005 Remove +12V and -12V add N1 and N2 Done Rev B
Mar-7-2005 Add input and output signals to bus so logic is not recreated card requiring IO.

Schematics, board layout, and operational description

Revision Production Date Schematic Layout Operational Description
A Last Documented Nov. 1982 Bus Definition Rev. A
B Updated Dec. 2005 Bus Definition Rev. B


System-4 Bus

The system bus card is built from VERO board and uses SIP style sockets and Radio Shack card supports. Each system card connects to the bus with a right angle single row header at the bottom of the card. At the back of the picture is the original power supply everything, is mounted on a piece of plywood. The AC power connector with a built-in switch was added last year.

System-4 6845 Alpha Numeric Card

The video card is based on a 6845 circuit from the May 1979 issue of Byte magazine. The circuit uses a 6116 2Kx8 Ram and provides dual port access to the RAM for both the system bus (circuit on the right) and video generation (circuit on the left). The circuit does not wait for horizontal or vertical blanking to access the RAM so the picture has a fair amount of “snow” while the video RAM is being updated. The monitor connects via a DB-9 hanging off the side of the card.

System-4 CPU Card

The system card is a fairly standard 1802 setup. A circuit remaps the 4K EPROM from the bottom of the address space (0x0000) to the top (0xF000) after the first access to an address in the upper block following a system reset. The circuit on the lower left side of the card replaces the original interface to an Apple parallel keyboard. A preprogrammed PIC PS2 keyboard decoder (PAK-VI) connected to a PIC 16F84 which acts as a serial to parallel converter connect to the original 1852 parallel IO port.

System-4 Memory Card

This is the top view of the 24K memory card, only 16K is populated. The jumpers on the card are for setting card location within the address space, the need for more memory never materialized.

The Bottom of the memory card. Back then building a computer was a little like knitting a sweater, the hours of handy work far out numbered the hours doing real engineering work.


This is the last of several 1802 based systems I built between 1978 and 1983. This system like all “real” computers has a system bus and separate Front Panel,CPU, Memory, and Video cards. The original system used an Apple parallel keyboard that was damaged many house moves ago. Last year the system was modified to use a PS2 keyboard everything else is original and seems to work :-). The system uses Tiny Basic or a modified version of the original Popular Electronics monitor program.