Picturing my old-fashioned Mini-Computer

05/29/2009

I have given a name to her: "MDM-24" for Multi-Dimensional-Matrix, 24 bits address. The internal architecture is more or less defined. But how will she look like from outside? --Like an old-fashioned minicomputer, of course!

The MDM-24 minicomputer is pretty much inspired by the legendary PDP-11. It consists of several rack-mount enclosures (units) interconnected through a common bus called UNIBUS. From outside, the UNIBUS is nothing but multi-pair cables ended with DB37 connectors going from one unit to another in a daisy-chain fashion. Each unit has its own power supply, of course.

The CPU unit distinguishes it self because of the Control Panel in the front with the classical switches and LEDs. Inside is the CPU Card but there are also Memory Cards and possibly others. Apart from UNIBUS connection, the CPU features a RS232 port (DB9 connector) used for maintenance only.

Other RS232 ports (DB9 connectors) are available in the "COMM Unit". Users connect to the computer using "Terminals" attached with serial cables to these ports. Since Terminals don't exist in modern times, users will employ PCs running Terminal emulation programs (such as HyperTerminal) instead. Remote users can also connect through the telephone line using modems as in the old days. Future development will provide Ethernet access to TPC/IP local network; Ethernet cards will also reside in the COMM Unit.

Storage goes separate in the "STORAGE Unit". It can host any sort of storage devices such IDE hard drives, RAIDs, flash, etc. Future development will provide access to USB Memory Sticks and external hard drives; those cards will also reside in the STORAGE Unit.

Other units can be developed in the future to meet specific needs. Also more units of same type (STORAGE for example) can be attached to the computer via UNIBUS. Even more than one CPU would be possible by structuring the appropriate arbitrage mechanism. Modularity at the Unit level (thanks to UNIBUS) is a key principle in MDM-24 design.

Units are also modular. They are basically multi-card frames with the UNIBUS in the back plane. So before producing a new unit, I will probably produce a new card for a existing unit.

There is no automatic boot up. When the MDM-24 is powered-on, she doesn't do much more than basic initialization of its micro-coded control circuit.

If the purpose is to experiment with low level software, this is the time for doing so. Software can be entered by hand using the switches in the Control Panel. It can also be entered using the CPU RS232 Port dumping large amount of code into selected zones of Memory or even the Matrix. By using the Control Panel, the computer can also be switched to Protected Mode back and forth, ran in single step, etc. This is "debug-time"!

If the purpose is to load the Operating System so the computer can do something useful, the operator has to enter the boot-strap routine manually, either by switches or CPU RS232 Port. The OS should reside in a well known place at the STORAGE Unit.

Once the OS is up and running, the computer can be operated from Terminals (or PCs running Terminal Emulation programs) attached to the COMM Unit (either locally or via modem).