This project aims to bring forward an MCS-51 based computer system centered around a series of relatively simple modules sharing a modular bus backplane.
The overall design uses a number of boards designed within the limitations of low-cost PCB prototyping houses and free PCB design software, i.e.
- Two signal planes (front and back)
- A maximum board size of 100mm x 100mm (yes, I know that some of them limit you to 100 x 80mm)
- A maximum of 300 connections per PCB
- A maximum of 2 signal (or groundplane) layers
Three card sizes have been defined and templates generated based on the 1U Eurocard -
- Full 1U Eurocard (100mm x 160mm)
- Half 1U Eurocard (100mm x 80mm)
- Short 1U Eurocard (100mm x 100mm)
These templates include placement masks for the bus connectors
Some of the simpler boards exist only as PCB files, in these cases, component values are marked on-board instead of component references.
The affected boards are:
- Modular Backplane (2 boards)
- 4-slot Backplane
- 5-slot Backplane
- Integrated Backplane (MOBO and daughterboard)
- Bus Daisy-Chain board
March 2024 After some consideration (including much fiddling about with wires and stuff), I have decided to redefine the bus layout.
It is still on a 40-pin and 34-pin plug, but I am recessing the card's connector and rearranging the pin-out somewhat in order to simplify some of the board layout hassles I have met with.
During the next update cycle, I will start to roll out these Rev. 2 designs.
- HMC-PR-1 AT89s51 Processor Board
The processor chosen for this project is the Atmel AT89S51 and AT89S52 microcontroller. These have 4k and 8k of ISP Flash Memory respectively.
The processor board is set up without user SRAM and is ready to accept firmware as a basic development board. The serial lines are buffered and brought out to a header (TTL signals) for connection to a user console. RS232 is available via the Front Panel board.
Console (serial) signals are also carried on the bus ready for embedded terminal hardware.
- HMC-MMB-1 HY6264A (8k x 8bit SRAM) x 4
- HMC-MMB-2 - Configurable W24257AJ (32k x 8bit SRAM)
Two versions have so far been designed
- one accepts a 32k x 8 fast SRAM chip, and is configurable in 8kB blocks.
- the second accepts up to 4 8k x 8 fast SRAM chips
Both versions may be located in upper OR lower memory space. Up to two of these boards may be installed.
Ultimately, these boards will accept
- UV-EPROM
- EEPROM
- FLASH PROM
And will provide storage for compiled programs (upper PStore) and System programs (lower PStore)
The decision was made to use low-cost, highly available standard 0.1" double row headers in order to reduce the board pin count while providing just a few spare lines above the bus minimum requirement. These are the same connectors formerly used for PC floppy and hard disk drives.
Although polarity is respected, just in case ribbon cable is used, it is actually maintained by the polarisation forced by the dissimilar connectors on the backplane.
- HMC-MB-1 (Slot A)
- HMC-MB-2 (Slot B)
The backplane comprises two boards, one carrying the 40-way bus that includes the default signals from the MCS-51 MPU and the other, a 24-way bus with additional signals and 5V power. This second board also includes decoupling capacitors per backplane slot.
The two boards are designed to sit side by side, and are able to be stacked, extending the basic backplane from the basic five slots to (theoretically) any number of slots. It is not recommended that more than two modules be daisy-chained without bus terminations.
A separate, hard wired 5.0V 2A (or more) power supply is required to power the bus.
M2 mounting holes are provided for attachment of lateral support bars (Grounded, recommend: 6mm square Aluminium, use M2 stainless washers to stand off from PCB) and cage sides (Isolated, recommend: 4mm acrylic). Half-holes are provided on the junction between the two boards in order to facilitate a third point of attachment to the support bars.
- HMC-IB-1 (MOBO)
- HMC-IB-2 (Bus)
This is a pair of boards that are stacked using extended pin header sockets.
The motherboard is a single 100mm square board which includes basic front panel functionality and power feed to the bus. A separate, hard wired power supply (5.0V 2A) is required.
The daughterboard provides a 4-slot bus powered via the motherboard.
It is recommended that this item not be daisy-chained without very careful consideration. The board is fused at 2A.
There is no RS232 console option on this PCB. This needs to be provided separately if required.
A low frequency clock "pulse" generator is provided for applications that require a low-frequency, semi-stable clock pulse for operation in the development stage.
The daughter board, built with standard sockets (i.e. not wire-wrap form) may be used independent of the parent board as a simple, no-frills 4-slot backplane. A mountable version of this PCB is available.
With only a plug and a socket, the daughter board could also be used as a slot extender for service use.
- HMC-PB-1
This is functionally identical to the 4-slot daughterboard for the integrated backplane kit, though with the addition of mounting holes.
- HMC-DCB-1 Short link for Modular Backplane use
- HMC-DCB-2 Long link for integrated backplane use
These board are supplied as two separate PCBs.
One narrow PCB for interlinking two Modular Backplane segments while maintaining system board separation. The wider is for linking an integrated backplane with a modular backplane segment.
Linking an integrated backplane with a modular backplane is not recommended other than for development purposes.
- HMC-ABT-1 While not strictly necessary, this short board is designed to be inserted in the first and last physical slots on the bus, and should reduce signal reflections and timing issues on the bus - especially when a longer bus is used.