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Old 14 November 2020, 19:47   #1
whitebird
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Join Date: May 2006
Location: France
Age: 45
Posts: 211
A1200 tower project

After years of occasional upgrades, please find my A1200 tower project:

Upgrades from standard A1200 are:

-Tower ATX PSU + power switch management
-PS2 keyboard support
-Hardware Buffered IDE interface
-Viper 1230/28 with 8 MB FAST RAM
-PCMCIA extender with front pannel fitted PCMCIA bay
-"self-made" Kickstart 3.1 chips
-20 GB 3,5" HDD
-CD-ROM drive
-PC floppy drive (HD not yet supported)

The aim was building a towered A1200 with as less money as possible. So, even if the configuration is not very powerfull, the financial goal has been reached as only the accelerator board has been purchased (20€ in 2004). All other components are recycling parts, even the motherboard. This one has been given to me as an accompanying gift by the guy who sold me the accelerator. It had some issues which I fixed over the past.
In order to minimize cost, the add-on boards have been made from standard prototyping plates.

The tower is a PC gateway 2000 case which I modified according to A1200 motherboard mechanical layout.

Please note the motherboard was not modified at all. The only add-on is a PIN soldered next to the PSU connector which serves as -12V input.

The built-in PSU connector has voluntary been left hiden to avoid voltage conflicts if by mistake an external PSU were connected.

The PSU panel switch latching circuit has been made from 2 74HXXXX ICs. At first, I tried a schematics from the web However, it did not provide satisfying debouncing, which led me to designing my own circuit.

As I did not find 2MBits 16 bits EPROMS chips compatible with Amiga sockets pinout, I had to use 4 8bits 128K Flash chips for upgrading to Kick 3.1. The ROM has been split into 4 files and programmed by the help of a PIC16F84 connected to a PC's RS232 port. The 4 chips have then been soldered to a prototyping plate where a hole has been made for 2 DIL sockets which have been glued. Wires from the Flash chips have been soldered on top of the sockets (were ICs are intended to fit) and the resulting board inserted in A1200 ROM sockets without any soldering.

The PS2 interface works on the same principle as the PC-Key 1200 (which does serial <=> parallel conversion) except it fits on the Keyboard MPU (through PLCC socket) instead of the white built-in keyboard connector. Synchronising the signals from the A1200 connector with a PS2 keyboard requires quite high reactivity in terms of CPU power. This may be the reason why PC-key 1200 requires a quite high MPU clock. Instead of Atmel, I use a microchip 16F747 MPU (40 pins) clocked at only 4MHz.

I always found the PCMCIA right angle adapter pretty useless as it requires opening the case each time a PCMCIA card has to be inserted. Moreover, it almost prevents usage of Wifi cards as the case acts as a Faraday cage against the EM signal.
As I had a dual PCMCIA bay from an old PC, it would be perfect adding this stuff to the front pannel, within the disk drive cage. However, the PCMCIA bay is intended to connect to a PCI board over two 40 pins IDE like cables. That didn't make things easy as I had to remove both IDE connectors at one end of the cables, and solder a female PCMCIA connector instead so that it connects to A1200's PCMCIA. Other challenge: neighboring pins on the cables didn't match with neighboring PCMCIA's pins. I thus had to rewire the whole cable in the middle.

The IDE buffered interface is RANDY 2. It normally cannot fit directly to A1200's MB unless some capacitors are changed or the board attached to an intermediate IDE cable. Hopefully I had some female to male 44 pins connectors which helped raise the board so that it does not conflict with the capacitors.

PCMCIA reset bug and slot disabling due to 8MB FAST RAM have been 100% fixed by software.

The Floppy drive has been taken from an old PC and modified for Amiga usage.

Upcoming upgrades: Self-made Scandoubler/Flicker fixer, High density support for floppies, USB ports add-on and maybe more powerful accelerator.

Please find below some commented pictures of the project and web browsing ability.
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