Clock Port Confusion

The “Clock port” is somewhat of an oddity. An internal connection on the A1200, it officially never existed in the form we use, but became so useful that you can add this strange row of pins to just about any other Amiga – and connect far more than just a clock to it. Still with us? Great. A history lesson for you, then….

In the early 1990s when the Amiga 1200 was a sketch on a drawing board, memory was expensive. Really expensive. Certainly in the UK it was common for offices to be broken into, PCs ransacked and all that would be taken would be the RAM modules. So the initial plan for the A1200 was to give it 1MB of Chip RAM, with the option of a second megabyte, to keep the costs down and this really is no different to the A500 and A600 where official Chip RAM expansions were produced and available at launch. What was different with the A1200 though is that Commodore decided not to occupy the trapdoor expansion for this – that would be reserved for Fast RAM and accelerators. Instead, two rows of headers were designed onto the mainboard to take a literal “piggy back” RAM board with the second megabyte on, directly on top of the on-board 1MB RAM.

Late on in development, memory dropped in price just enough to make it feasible to give the stock model 2MB at launch – and as that’s all that the AGA chipset could address, there was no reason to keep those headers intact as more Chip memory couldn’t be added. But, the plan all along was for some of the now redundant pins to be left in place, even on a 2MB machine, as the schematics contain “Only this bit of P9A/B loaded for 2Mbyte PCB” or “only pins 19-40 loaded for 2MB configuration”.

So clearly Commodore had plans for it all along, but why? The A1200 user manual makes no mention of it at all, guiding you towards the trapdoor and PCMCIA ports for any upgrades you may want to fit. A possible clue is very close to the pins, on the mainboard – an area of unpopulated components next to the RAM that could have taken a real time clock is still present but never used. It’s likely that Commodore decided to save a small amount on the build price of each machine, by leaving off the clock components and instead providing a port to connect an aftermarket one to – and half of the lower section of the once-Chip RAM expansion port provided those signals needed. This presumably would have meant the “official” 1MB upgrade would also have included a clock.

So we are blessed with a small port that connects directly to the address and data bus. We call it the Clock Port as that was the main use (and possibly the intended use) initially, but with that type of direct bus connection it was inevitable that other devices could be designed to work and now we have USB controllers, sound cards, networking and even RGB lighting controllers. It also sounds better than “the right half of P9B”.

But wait! There is something important to know. You’ll be unsurprised to hear that Commodore didn’t get this quite right, and a small but not insignificant number of revision 1D.1 mainboards have this port soldered to the wrong side. Our repair service sees one every few months, and spends the time to correct it. There is a really good reason for that, which is that it’s theoretically possible to somehow jam a Clock Port device onto the “wrong” pins (especially when it connects with a cable, such as a RapidRoad) and destroy the mainboard, the accessory, or both. Spot the difference in the images below.

There are also more boards, early revisions such as 1B, with a full width connector on that have a similar risk but at least are usable if you’re careful. You may have to snip pins 17 and 18 to get a cable on, though.

You’ll also have to be very careful connecting accessories. Since the remaining pins are the 22 pins from 19-40, there is no “pin 1” so different manufacturers of add-ons have different ideas where the red stripe should go. Always always triple check orientation of cables against the manufacturers instructions, and if in doubt – ask. The resulting damage isn’t trivial!

Leave a Reply

Your email address will not be published. Required fields are marked *