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On the Amiga 500 there is an expansion port

Amiga 500 Expansion port

How did the expansion port work?

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  • Note that the IBM PCjr had a similar type of expansion system, on the right hand side when facing the front of the computer, as seen here: oldcomputers.net/pics/ibm-pcjr-sidecar.jpg
    – Todd Wilcox
    May 9, 2016 at 21:03
  • With all due respect, this is much too broad a question to be really useful. It should probably be rephrased in terms of what the uses of the expansion port are rather than "how it works", which is a topic which require an entire book. It could also be split in two: - what can be done with the expansion port (practical) - which signals does it expose (technical)
    – Laurent Giroud
    Jul 3, 2016 at 1:19
  • Hi Laurent, i don't think this is too broad. However, I do think my follow up question in the post is. I'll remove that.
    – Thraka
    Feb 8, 2018 at 22:43

4 Answers 4

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That expansion port exposes all the signals from the 68000 CPU, plus some other Amiga specific signals, such as the color CLK, ROM and RAM select signals, along with voltage supply lines, etc. It is documented in the schematic, page F-9 of the User's Manual.

It worked by removing its trap door, and plugging in a compatible device with the Amiga switched off. When switched back on, the device starts operating and depending on whether it has a boot ROM, it may override the normal Amiga boot procedure, or simply sits there in the bus waiting for a driver that had to be loaded during the Amiga OS initialization process so it can answer requests from the OS and applications.

You can put it any hardware device designed to work with the 68000 bus and the Amiga 500: from an IDE interface to a complex device. Or you can completely override the internal 68000 and provide your own CPU, as long as it is binary compatible with the 68000 and generates the same control signals (e.g. what an accelerator board does)

Common expansions for the A500 included: Action Replay, SCSI hard disks and CPU accelerators. The Amiga Hardware Database lists many expansion devices, many of them usable with the A500 through its expansion port (along with other devices that used the parallel port or the memory expansion port) http://amiga.resource.cx/expansion.html

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    The bus is also pass-through compatible. Many expansions included a second expansion port on the opposite side of the device allowing devices to be daisy chained.
    – mnem
    May 9, 2016 at 21:09
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    One of expansions was a really cheap 8MB RAM expansion. Of course you had to provide the expensive RAM chips (SIMM, AFAIR) yourself :)
    – SF.
    May 10, 2016 at 13:55
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    @SF The most common of the sidecar RAM expansions used ZIP (Zig-zag in-line package) RAMs. I have a SupraRAM 500RX expansion on my Amiga 500 and this is what it uses, up to 16 ZIPs to make 8MB of RAM. Mine was populated with 2MB of RAM when I bought it, a pretty common stock configuration. link
    – mnem
    May 11, 2016 at 8:10
  • If this connector exposes the signals of the internal CPU, that obviously allows all sorts of things to be done with the bus, adding peripherals, snooping, etc.... but how exactly does it allow the user to override that CPU with a different, external one?
    – underscore_d
    May 25, 2016 at 21:33
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    @underscore_d : see the answer to this question. retrocomputing.stackexchange.com/questions/443/…
    – mcleod_ideafix
    May 26, 2016 at 6:40
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In addition to mcleod_ideafix' answer above: Commodore also made a CD-ROM drive that plugged in the slot (unfortunately without a feed-through slot on the other side): the A570 turned the Amiga 500 into a CDTV.

Unfortunately, the A570 was released after the A500 was discontinued and replaced with the A600, it had a caddy (not a tray) and it ran at single-speed. So it wasn't very popular.

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  • This capability is already included in the "from an IDE interface to a complex device" sentence, moreover the last paragraphs points to a site which lists that CD-ROM drive.
    – Laurent Giroud
    Dec 23, 2017 at 22:55
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In short, the Amiga 500 expansion port was an exact electrical equivalent of the expansion port of Amiga 1000, and very similar to the Zorro II card slots of the Amiga 2000 and above.

The Zorro II card slots of the Amiga 2000 and greater models had 14 extra pins compared to the Amiga 500/1000 expansion port.

See: http://www.ntrautanen.fi/computers/hardware/misc/amiga_zorro.htm

When you remove the expansion port cover on an Amiga 500, the large chip you see above the card edge connector is the actual 68000 processor - so you can see that the expansion port provides very direct connectivity to the processor.

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Another fairly common expansion, which was not an accelerator, RAM, or 68k compatible, was the PC compatibility card containing an x86 compatible CPU. In fact, one of the first hyped Amiga sidecar expansions was the original Sidecar.

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