There were four major revisions of the Amiga 500 motherboard released, and that matters if you are trying to upgrade them. Fortunately, if you know what to look for, you can quickly distinguish between all four, and you can even identify them without opening the case. Although it is certainly easier to identify them if you can see the entire board.
Which Amiga 500 motherboard revisions need an EPROM adapter
I don’t know about you, but the main reason I cared which Amiga 500 motherboard revision I had was so I could upgrade it. Amiga 500 Rev 3 and Rev 5 motherboards do not work with Kickstart burned to an EPROM as they came from the factory. You need to add resistors or use an adapter to use Kickstart on an EPROM. Or you need to use a mask ROM.
The later revisions, Rev 6A and Rev 8A, work fine with Kickstart on an eprom.
Keep in mind that even when you buy a Kickstart ROM from an authorized distributor these days, it usually comes on and eprom. Kickstart ROMs are not produced in sufficient quantities to make mask ROMs cost effective anymore.
FAB 312512 is a red herring
Don’t make the mistake I did. The artwork number or FAB number on the board, 312512, does not identify the motherboard revision. All four versions of the motherboard have that number on them.
To figure out which Amiga 500 motherboard revision you have, you need to look for something other than the assembly number.
The best identifying marks are frequently under the floppy drive in an assembled Amiga 500, but you can still identify an Amiga 500 motherboard without taking it apart, or even opening the case.
Identifying an Amiga 500 Rev 3 motherboard
The Amiga 500 Rev 3 motherboard has several distinguishing characteristics. To identify it, you don’t even have to open the Amiga 500 case. Simply pop open the door on the left side that covers the expansion connector and look at the expansion connector. On a Rev 3 board, the leftmost pin on the connector is double the size of the rest, because pins 1 and 3 are electrically connected.
This is the fastest way to identify if a revision 3 or revision 5 board is in an Amiga 500. If pins one and three on the expansion connector are joined, that board needs an EPROM adapter.
To distinguish between a Rev 3 and a rev 5, look behind the pins. If you see two vias behind pins 1 and 3 but not one behind pin 5, it’s a rev 3.
Identify a loose Amiga 500 rev 3 motherboard
To identify a loose board, you can spot a Rev 3 at a glance without needing to look at the expansion connector. Look at the bottom edge of the board. Revision 3 and revision 5 motherboards have 16 RAM chips and all of the RAM chip positions are populated. So if you see 16 ram chips, it’s either a revision 3 or a revision 5 board.
Distinguishing between a revision 3 and a revision 5 is easy. There is an identifier right above the RAM chips. On a revision 3 board, that identifier is etched into the copper. The text etched into the copper identifies the board as a B-52 Rock Lobster and has the names of the designers. That identifier was moved to the far right hand side of the board under the floppy drive in later revisions.
The rev 3 board came in the earliest Amiga 500s and it’s not as usable as later boards. It’s buggier and it can’t be upgraded to 1 megabyte of chip RAM, so it has many of the same limitations as an Amiga 1000. But it may also be interesting as a collectible, for the same reason as early Commodore 64 boards.
Identifying a revision 5 motherboard
Identifying a loose revision 5 board is easy at a glance, whether in person or from photos from an auction site. First, look at the RAM chips. A revision 5 board has 16 RAM chips, with all of the positions populated. Above the RAM chips, you see a silkscreened identifier in white that says A500. The combination of 16 RAM chips and the white identifier is the telltale sign of a revision 5 board.
You can also identify a revision 5 board without opening the case.
From the trapdoor, an Amiga 500 revision 5 motherboard looks very much like a revision 3 motherboard. Pins 1 and 3 are connected just like on a revision 3 board. But there is one subtle but noticeable difference. The key is to look at the vias behind the pins. If you see vias behind pins 1, 3, and 5, it’s a revision 5 board.
Identifying an Amiga 500 Rev 6A motherboard
The Rev 6A motherboard had a number of easily identifiable changes from earlier revisions. All of the pins on the expansion connector are the same size. on both a rev 6A and 8A board. So if you open an Amiga 500 expansion door and see all of the pins are the same size, that motherboard can use an EPROM as a Kickstart ROM without modification or an adapter.
To differentiate between a 6A and 8A, you have to look closely. Behind the pins, look for an identifier that says JPL between pins 7 and 9. Rev 8a boards will have identifiers that say E702, E703, and E704 in that space instead.
To identify a loose rev 6a motherboard, look at the RAM chips. The Rev 6A board used higher density chips, so it only took four chips to get 512K of RAM. There are positions for an additional for chips to expand the machine to one megabyte of RAM on the motherboard, although they didn’t come from the factory with those positions populated.
The silk screen on the motherboard above the RAM chips on a revision 6A board says A500. So this makes rev 6 boards easy to identify: Just look for the white A500 silkscreen and 8 RAM chip positions rather than 16 to distinguish it from a rev 5 board.
Amiga 500 rev 8A motherboard
The Amiga 500 Rev 8A motherboard was intended for the Amiga 500+ model. But the board will fit in a regular Amiga 500 case, because the only difference between the 500 and 500+ case was the badge. So you can’t just go by the case badge, because boards and cases could have been swapped over the years.
When you open the expansion door on the side, on a revision 8A motherboard all of the expansion pins are the same size.
Behind the pins, look for identifiers that say E702, E703, and E704 near pins 7, 9, and 13. A Rev 6a board has a single identifier that says JPL in that area.
That’s the easiest way to distinguish a revision 8A motherboard without opening the case.
Identifying a loose revision 8A motherboard
If the case is open, or the board is loose, it’s really easy to identify a revision 8A motherboard. The silk screen near the bottom of the board above the RAM chips says A500+ rather than a 500. So if you see the silk screen with A500+, it is a rev 8A board.
Like the Rev 6 board, the rev 8 board has eight positions for memory chips. Normally, all eight chips will be populated, but this isn’t a foolproof way to identify between a Rev 6A and 8A motherboard.
The other major distinguishing characteristic of a rev8 a motherboard is a position for a clock chip and a battery near the trap door expansion connector. If you see a battery on the motherboard itself, not on an A501-style expansion board, that’s a good indicator of a revision 8A motherboard. If it’s a nicad battery, snip it out, and I hope it hasn’t leaked and damaged the board yet.
Why it matters
If you are restoring an Amiga 500, you might like to know how original it is. Identifying the motherboard can help you to determine if the machine has been modified.
The revision 6A and 8 a motherboards are desirable because they work with the ECS chipset. This matter is less if you are playing games on your vintage Amiga, but if you have a need for ECS, you want one of the later motherboard revisions.
The main reason I cared which motherboard I had was because I wanted to upgrade the machine. I wanted to install a Terrible Fire accelerator that included IDE, and to run a Terrible Fire at full speed and use the IDE port, you need Kickstart 3.1. I have vintage original 1.3 and 2.1 ROMs, but I don’t have a vintage 3.1 ROM. So I needed to buy a modern one, so I needed to know whether I needed an adapter as well.
A Terrible Fire doesn’t turn an Amiga 500 into an Amiga 1200 by any stretch. But it does make an Amiga 500 much more usable. Using WHDload takes care of backward compatibility issues, so any software that runs on classic 68000 Amigas becomes available to you on a standalone system.
An Amiga 600 takes up less space, but a Terrible Fire lets you run a 68030 based Amiga 500 that will keep pace with an Amiga 3000 or even outrun it, while taking up the same space as an Amiga 500.