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Let’s build a memory expansion board with RTC functionality for the Amiga 500 and 500 plus.
I found it by chance while looking for a way to test the trapdoor of an A500+, and after that I had trouble finding the post and the components to build it again, because they were spread over several messages. I decided to write a summary so that everything can be found in a single article.
The board was designed by PeteAU and you can find his post at this link on eab.abime.net
You will also find the gerber to download, which for simplicity I’ve added below:
Schematic rev. B6 in Eagle
Gerber miniA501b6 uses SOJ42 1M x 16 memory chips.
Gerber miniA501c6 uses SOJ40 memory chips, no other difference.
This link to see his page with the compact version of the project.
Here alongside is the PCB of the main board; yes, I say main because together with this gerber you can also print an adapter to fit under the Gary chip on the Amiga 500+ to expand it up to 2MB.
In this article we’ll see how to build it and the components needed for the 512KB memory expansion function for the A500, 1MB for the A500+ and RTC for both versions. The model I built is for SOJ42 memory chips, so we’ll be talking about the rev B.6 gerber
Here are the components you need for the memory expansion:
U1-U2,U3,U4: AS4C1M16E5-60JC (one is enough for the A500, two for the 500+)
Fit at least 1 100nF capacitor for each memory chip; the capacitor pads are both above and below the DRAM and not all of them have silkscreen markings.
1×3 header for the second memory chip (U2 A500+)*
1×2 header to disable the expansion (A500)*
2×40 90° header, or 2×56 90° if fully populated with RTC
These are to be added for the RTC section:
U5 o U6: RTC62421/RTC72421/RTC62423/RTC72423
C10 or R7: 47K~100K 0805 resistor (that’s right, a resistor!)
C12: 100nF 0805 capacitor
C13 or C19: >=2.2μF >=10V 0805 or 1206 tantalum
R1, R2, R3: 10K 0805 resistor
R4, R5: 220~470 0805 resistor
D1: BAT721C or BAT54C diode
Coin cell battery holder
1×2 header to disable the RTC section*
*optional
Let’s get started
As seen above, there aren’t many components and we can use several alternatives.
If we only want an expansion for the A500, a single U1 memory chip, one capacitor and a connector of at least 2×40 pins will do.
To use the RTC as well, the connector must be full length; if you can’t find a 2×56, you can take smaller ones and join them, or a longer one and cut it down.
The jumpers are optional if we don’t want to change how the board works, but if we want to disable some functions or also configure it for the Amiga 500+, then they will be needed and we will have to cut the bridge already on the PCB.
Let’s take the example of building a complete board with RTC and 1MB for the A500/500+:
we need to fit 2 memory chips, U1 and U2, with their capacitors, solder a 1×3 strip on the RAS0-RAS1 pins above U2, cut the bridge between the clock pins and the enable pins to fit a jumper there, and also solder the components for the RTC section.
As usual it’s best to start with the small components, but since we’re talking about SMD and we’ll only flip the board over once for a few capacitors, we can do whatever suits us best.
Operation
Now that the board is complete, let’s see how it works in different configurations.
Clock jumper:
if left open, these pins disable the RTC section and our Amiga won’t see the module connected; useful if we are using the 500+, which already has an RTC on board so it isn’t needed, although since its battery does a lot of damage it is usually removed.
Enable jumper:
These two pins enable the memory expansion board, but only for the A500; on the 500+ the expansion is seen anyway.
RAS0-RAS1:
These jumpers above the memory chips are used to configure which line they communicate on. The first memory chip already has the bridge on RAS1, so to use 1MB on the A500+ we need to set U2’s jumper to RAS0.
In one case I had to cut U1’s jumper bridge and move it to RAS0 to get it working properly on an Amiga 500.
Other bridges around the board are for use with the interface under the Gary.
Below is a summary of the configurations:
| Function | Enable | Clock | U2 RAS0 | Amiga |
|---|---|---|---|---|
| 0Kb | open | open | open | A500 |
| 512Kb | close | open | open | A500/500+ |
| 1Mb | close | open | close | A500+ |
| 0Kb RTC | open | close | open | A500 |
| 512Kb RTC | close | close | open | A500/500+ |
| 1Mb RTC | close | close | close | A500+ |
All of its functions?
As well as what we’ve seen, if completed with the PCB that fits under the Gary, this board can do all of this:
Expands 0.5MB SLOW/CHIP or 1MB CHIP RAM on the A500 (Agnus 8372 mod required)
Standard: adds 1.5MB of SLOW RAM on Amiga 500 rev. 5/6; rev. 3 requires motherboard modifications.
Adds 0.5MB as CHIP RAM together with the SLOW RAM, or up to 2MB with a modification to the A500 board.
Don’t miss the video of the build and the hands-on test on the computers.
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2 Comments
Buonasera, ho assemblato questa stessa espansione per un A500 ed ho il problema che la batteria dura pochi giorni poi si scarica.
AmigaTestKit rileva sia la memoria che l’RTC ed i test di memoria non evidenziano problemi. Ho controllato i vari componenti e mi sembra tutto ok, cosa altro potrei controllare? Grazie
Non saprei di preciso, lo schema non l’ho realizzato io ma non ho avuto problemi di questo tipo. Io controllerei prima quando assorbimento c’è sulla batteria con un multimetro in serie e misurando i mA che passano, sia a computer acceso che spento per capire se il problema è un consumo alto costante o se quando è collegato si scarica più facilmente per via di altri problemi. Poi da li cercare di capire se fosse il diodo di protezione o l’integrato rtc o qualche componente o magari un pin o flussante in giro che fa una bassa resistenza.