Open Hardware by SukkoPera 1 MB Chip RAM Expansion for the Commodore Amiga 600 Computer with optional Real Time Clock.
Summary
The Amiga 600 computer by Commodore originally came with 1 MB of Chip RAM. Like all “compact” Amiga models, it has a trapdoor in the bottom, through which a memory expansion can be installed, bringing the chip RAM to a maximum of 2 MB.Back in the day, some expansions also came with a Real-Time Clock (RTC), which allowed the Amiga to keep track of the current date and time, which is most useful if you use AmigaDOS a lot. While this was mostly ordinary for Amiga 500 expansions, it was not so common on those for the Amiga 600, and even most expansions designed today do not bear an RTC circuit.
OpenAmiga600RamExpansion is an Open Hardware implementation of such an expansion, including the RTC.
Once you have maximized the Chip RAM in your A600, you might be interested in adding some Fast RAM as well: please check out OpenAmiga600FastRamExpansion in case.
Schematics
Components
Connector
The female connector for the male edge connector on the A600 PCB is probably the hardest part to get hold of. Many people use PCI connectors, which have the correct pitch but are longer and need to be trimmed. They are also hard to solder, most of the time. This project uses a connector that is normally used with the micro:bit educational computer instead. It was discovered by chance to be a perfect fit for the the A600, cheap and reliable.
Memory
The required RAM Type is 4 Mbit (256k×16) DRAM in the SOJ-40 package with up to 70-80 ns access time, e.g.: check list
Normally it is not necessary to mount all the decoupling capacitors. I usually skip C2 and C4. All of them are 100nF in the 0805 package anyway. An additional 10uF electrolytic capacitor can be mounted at C9, if needed. Might be a good idea if using a Vampire board or a power-hungry machine.
RAM chips can either be soldered directly on the board or installed in sockets. While soldering the chips might not be trivial for the unexperienced, sockets for the SOJ-40 package are quite expensive and not really easier to solder either, so the choice is up to you.
RTC
The entire RTC circuit is optional. It has been designed for an Epson RTC62421 or RTC72421, which don’t need an external crystal nor any calibration. Both can be bought very cheaply from China: they will most likely be second-hand “pulls”, but usually they will work fine.
The other components have some degree of flexibility:
C7: 100 nF 0805 capacitor
C8: >= 2.2 μF >= 10 V 1206 tantalum capacitor
R1, R2, R3: 10K 0805 resistor
R4, R5: 220-470 0805 resistor
D1: BAT721C or BAT54C diode
Battery Holder: Some cheap eBay one, usually called BS-7
Battery: CR2032 3V Non-Rechargeable Lithium (a new one will last years)
Note that if you plan to shelve your Amiga or not to use it for a while, you are recommended to remove the RTC battery. Failing to do so killed many A500+ computers and many A501 expansions in the past. The CR2032 battery used in this project is probably safer than the barrel batteries used in those devices, but still I would not leave one in an unused Amiga. You have been warned.
Installation
After everything has been soldered, just open the trapdoor on your Amiga, install the expansion (the correct orientation is with the chips closer to the keyboard) and put the cover back on.
I recommend to run Amiga Test Kit (formerly SysTest) then. Use the Memory option (F1), it must show 2 MB of Chip RAM. Then start the Memory Test (F1 again) and let it run for 50-100 rounds: if it doesn’t find any errors, you are probably good to go.
Amiga Test Kit also has an option for reading and resetting the RTC (F7 then F3), so try that as well. If you are using Amiga Test Kit or SysTest >= 1.2 you can also set the date and time with it, otherwise you will have to use other tools (Workbench is a good candidate).
Issues
The board has been thoroughly tested and it works perfectly, but if you are trying to understand how it works, please be aware that there are some oddities in the schematics.
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