The OpenKickstartSwitcher in combination with the OpenAmigaMouseTrigger makes a great enhancement for your A500/+. These fantastic little pieces of hardware - made by SukkoPera - in combination with a 27C160 EPROM makes it possible to switch between 2-4 different KickStart (KS) images. If you add a second trigger source, you could switch between 2-6 KS images. The maximum numbers of images you can use in you switcher depends on what EPROM you use:
- 27C800 can hold 3 images - 2x 256KB and 1x 512KB.
- 27C160 can hold 6 images - 4x 256KB and 2x 512KB.
Choice of trigger signals
In the following example I will use one OpenAmigaMouseTrigger, and a 27C160 EPROM populated with 6 images. However, I will only switch between 4 of them; the other two will just be for filling out the EPROM so that it can be programmed correctly.
When using a larger EPROM like 27C160, you have to use the bottom pad on the KS Switcher - A19 - to be able to select all 6 positions. You will also need a second Mouse Trigger, a physical switch, or some other source that is possible to set a logic LOW signal for A19.
Because I only use 4 images on my switcher, 2x 256KB and 2x 512KB, I will use SW2 in combination with A19 to get my desired combination. Also the SW1 and SW2 has pull-up resitors, so if no signal is connected it will always stay HIGH. This way I can skip SW1 and get away with only one Mouse Trigger. Same goes for the direction signals from Joystick 2, center postion = HIGH.
The truth table can be a bit confusing as the selection order gets reversed compared to the order you put your images in when programming the EPROM.
See how I populated the KS images in the truth table below, X = do not care:
|ROM Image #||Size - KB||(SW1)*||SW2||A19|
|1 - Fill out KS image - Not able to select||256||(LOW)||LOW||LOW|
|2 - DiagRom 1.2 (truncated)||256||(HIGH)||LOW||LOW|
|3 - KS 3.X||512||(x)||HIGH||LOW|
|4 - Fill out KS image - Not able to select||256||(LOW)||LOW||HIGH|
|5 - KS 1.3||256||(HIGH)||LOW||HIGH|
|6 - KS 2.05||512||(x)||HIGH||HIGH|
*not used in my case
Translated to my trigger signals, the selection will be as follows:
6 - KS 2.05 - Joystick centered (default image)
3 - KS 3.X - Joystick Right
5 - KS 1.3 - Joystick Up
2 - DiagRom 1.2 - Joystick Right + Up
Note, if you are only using the 27C800 there is another truth table to be used, see below:
|ROM Image #||Size - KB||SW1||SW2|
A common source to use as trigger for a KS switch is fire0 - left mouse button, or fire1 - Joystick 2, button 1. These signals could be easily be hijacked at the vias nearby the ODD CIA chip, see image below (rev. 6A).
I choose to go for a different approach, because I already had my mouse buttons hooked up to alternative startup-sequences, and I needed to be able to switch between four different KS images.
The signals I picked are the Right and Up direction of Joystick 2, they could be found under the disk drive.
However, as long as you choose two adjacent directions you can pick any pair you like to achieve your four trigger signals, If you need less than four, you can pick any signal you want.
Mounting of the KS Switcher and the Mouse Trigger
When handling ICs, it's advisable to use an electrostatic discharge (ESD) wrist strap to ground yourself to the PCB. This is to be sure you do not damage the sensitive ICs.
When you have decided which trigger signals to use, it's time to heat up your soldering iron!
Use a solder wick to clean the vias from solder, don't forget to use flux to make the process easier. Sometimes you will need to use the solder wick on both sides. My favorite is Chemtronics 60-2-5 Soder Wick No Clean SD, it's a bit pricey but totally worth it IMO!
There is a suitable hole for the wires in the "ARTWORK" silk, and there is a matching hole that goes through the shielding as well. Depending on your insulation layer, you might have to make a matching hole get the wires through, fortunately mine was already there. Also, there is a perfect place for the trigger to the right of these holes, between the lower mounting points of the disk drive.
Be sure to color code your wires, so they have the same color for in and out signals, it will make your life a bit easier! The Mouse Trigger holds in place by itself, thanks to the wires going through the hole in the shielding and the plastic support for the disk drive.
To remove the KS ROM from its socket, use a small flat screwdriver. Insert the screwdriver on one side and carefully twist it a tiny bit clockwise. It is important that you do not lift the pins from the socket all at once, if you do you might bend and damage the pins.
Repeat this procedure, but every second time you change the direction you twist the screwdriver - this is to lift the ROM from the socket in all four corners.
When it feels like the ROM is about to come loose, grab it with your fingers and carefully wiggle it until it is completely loose. If it too hard to wiggle, go back to the procedure with the screwdriver.
Inserting the adapter is easy, just push it carefully down into the KS socket.
I missed some photos during my build, but I think you get the picture anyway!
Also, I can warmly recommend you to take a look at SukkoPera's other open source repositories, there is a lot of nice retro stuff there!
One of the cooler projects is his Reamixx500, a fantastic effort to try and save the A500 for future enthusiasts!
I hope you enjoyed all this, and please stay tuned for more interesting posts 😉