Amiga 3000 ROM Bank Switcher (ROM Tower Version)

TLDR: Yes, you can use this board to add standard 27C400 EPROMs, such as those programmed with AmigaOS 3.1.4 or AmigaOS 3.2 in your older revision Amiga 3000 where only the U182/U183 sockets may be used. Yes, it replaces the ROM Tower, which might or might not have come with your Amiga.

The Amiga 3000 ROM bank switcher (ROM Tower Version) is a simple board which allows the installation of 2x 2MB ROM/EPROM/EEPROM in the Amiga 3000 Kickstart ROM sockets U182 and U183. These sockets only support 256KB Intel 27C220 ROMs (X 2 sockets = 512K Kickstart image), but with this bank switcher, you can use the more common 28F400/28F800/28F160 ROM types which would typically be installed in U180/U181 in newer Amiga motherboards. The ROM Bank Switcher provides jumpers which allow the other space of the ROMs to be used as alternative Kickstart images. This switcher is designed for the earlier Amiga 3000 motherboards (before 9.x) where the ROM Tower is required for the 27C400 EPROMs to be used. Not all older Amigas came with the ROM Tower. The ROM Bank Switcher can be used in place of the Commodore ROM Tower to allow the same functionality of the ROM Tower. This ROM bank switcher would probably work with newer Amiga 3000 motherboards as well. Unfortunately, most of them do not have the U182/U183 sockets populated. DO NOT INSTALL THIS BOARD IN U180/U181. It does not work there, and could damage your Amiga and/or your ROMs. If you have a Rev 9.x Amiga motherboard, use the Amiga ROM Bank Switcher 3000 in those sockets. You may also be interested in the Amiga ROM Bank Switcher board for other Amiga models.

The A17 jumper supports three positions:

• No Jumper - select the Upper 256KB image
• Left (Low) - select the Lower 256KB image
• Right (Bus) - let the Amiga control this address line (note that you will need to run a wire from the CPU A19 signal to the single Bus A17 pin on the switcher).

• No Jumper - select the Upper 512KB image
• Left (Low) - select the Lower 512KB image
• Right (Bus) - let the Amiga control this address line (note that you will need to run a wire from the CPU A20 signal to the single Bus A18 pin on the switcher, and other modifications may be needed as well).

• No Jumper - select the Upper 1MB image
• Left (Low) - select the Lower 1MB image
• Right (Bus) - let the Amiga control this address line (note that you will need to run a wire from the CPU A21 signal to the single Bus A19 pin on the switcher, and other modifications may be needed as well).

Intended use

Some Amigas (A1200 for example) allow for 1MB ROMs instead of 512K. This is unfortunately not the case for the Amiga 3000. There, the limitation is both the A3000 Gary and also the U182/U183 sockets. The Gary issue can be solved with an enhanvement (Romy -- more on that below). The U182/U183 socket issue is that ROM A17 is not available at those sockets. ROM A17 is needed to address more than a 512KB Kickstart image. So to use, for example, a 1MB Kickstart image, you'll need to connect Bus A17 on the ROM Tower / Bank Switcher to CPU A19 somewhere on the Amiga. Gary is a good choice for this. When using a 1MB image, you'd then set the A17/Bus jumper so that the ROM A17 would be driven by the CPU's A19 signal.

Instead of a switch or switches on the exterior of the computer, another option is to just use two-pin jumpers, placing one each as desired on the A17, A18, and A19 headers. This won't make changing the selected ROM image very easy, but you may only want to use a single ROM image anyway.

Board source

The Gerbers are available in Github here: https://github.com/cdhooper/amiga_rombankswitcher_a3000_romtower

Assembly

• 4x 20-pin male-male machined pin headers, which you might need to cut to length. BE SURE to get machined pin headers -- do not use square pin headers as these will damage your motherboard socket.
  eBay search example - Amazon search example
• 2x SMD 0805 100nF capacitors.
• 2x 42-pin 2.54mm DIP IC sockets, preferable machined-pin. These are a little more difficult to find. eBay search example - I wasn't able to find these on Amazon. You could certainly find them on Digikey or Mouser
• 3x SMD 0805 10k resistors. You can find these on eBay or Amazon with the "10k SMD 0805" search term. eBay search example - Amazon search example
• 4x right-angle 2-pin or 3-pin headers. You really only need 2-pin headers for A18 and A19, as most likely you won't be modifying your Amiga further to support 2MB ROM space. It's probably cheapest to buy a single long header and then cut to length. eBay search example - Amazon search example

1. Install the three SMD resistors and two capacitors on the bottom
   If not using hot air, the easiest way to do this is use your soldering iron to put solder on one pad, then use tweezers to lightly hold your component in place and use the soldering iron to solder down that side. Make sure the component ends up flush with the surface. Then you can set the tweezers down, pick up the solder, and move to the other side, applying solder with the iron.
2. Install only the U182 bottom pin headers
   A simple way to keep the pins aligned as you solder them down is to temporarily install them in your 42-pin socket (long side of the pins go in the socket) and then solder them down to the PCB. You can then remove the socket after they are fully soldered.
3. Install the top U181 socket
   Align the notch of the socket with the notch shown on the silkscreen.
4. Install the U183 bottom pin headers
5. Install the top U180 socket
   Align the notch of the socket with the notch shown on the silkscreen.
6. Install the three jumper headers on top
It may be tricky to get these to stay exactly where you want them. I found it's easiest to just tack down one pin, then use tweezers or your fingers to manually stick the header where it should go as you heat the pin. Another way might be to use tape to hold the headers in place while you solder.
7. Optional: Connect Bus A17 to the CPU's A19 address bus pin. This will allow you to use 1MB Kickstart images even though the U182/U183 sockets do not support these. There are several places on the board where you can pick up A19. A common place where people attach is Gary pin 15, but it's not particularly easy since you need to locate the pin of a PLCC package. You might find RP107 pin 6 or U704 pin 7 easier to reach. Closest would probably be to use the CPU A19 in the U180/U181 sockets, but you can't rely on the schematics to find this trace, since the reason for not using U180/U181 (on boards below Rev 9.x) is that Commodore's footprint for these sockets was incorrect and they weren't routed correctly.

Another view of the same board on the left.

...and a top view of the same board on the right.

On the left, I've tacked down the corners of the four header sockets. I can then pick up the board and press the headers to the PCB as I heat up the tacked pins. This allows the opportunity to make the headers flush to the PCB.

On the right, the headers have been fully soldered down to the board.

Now remove the sockets from the bottom headers and install them in the top (left).

On the right, these headers have been soldered down.

Finally, install the jumper headers. As you see here, I used a three-pin header for Low/17/Bus and two-pin headers for Low/A18 and Low/A19 positions. You could choose to install all 3-pin headers if desired, but as stated above, it's likely you will not use the Bus positions of the header for A18 and A19.
If you plan to use 1MB ROMs, you will need to run a jumper wire from the Bus A17 header (you could also solder it down instead of using a header if this board is going to be permanent in your Amiga). This wire must connect somewhere else on your motherboard where CPU A19 is provided (Gary is a good choice), since it's not available at U182/U183. This is not a mistake. ROM A17 connects to the CPU's A19 signal on the Amiga 3000.

To use 1MB ROMs in your Amiga 3000, you will also need an enhancement to Gary which decodes the second half of the 1MB ROM. Look for the Romy project by SpeedGeek. A much improved version of Romy can be found at https://github.com/reinauer/amiga-romy by Stefan Reinauer.

You are done with assembly.

Installation

Press very slowly, ensuring all pins which are visible around the edge of the board go properly into the Kickstart ROM sockets. I've found that using a magnifier helps a great deal with this task.

You may choose to install your two Kickstart ROMs/EPROMs/EEPROMs before or after the Kickstart switcher is installed in the Amiga. The U180 ROM might be marked as "Low" or "ROM-1". The U181 ROM might also be marked as "High" or "ROM-0".

When done, you should minimally test that there isn't a dead short between GND and 5V with the switcher installed. The easiest way to do this is to check for continuity between pin 22 of either socket and GND of the Amiga (any metal contact from the chassis). If you see less than 5 Ohms, there is a problem (do not power on). Pin 22 is located in the bottom right corner of the two chips, if you look at them from above, with the notch facing to the top (Pin 1: top left corner, Pin 21: bottom left corner, Pin 42: top right corner).

Before fully reassembling your Amiga, I recommend doing a test power-on to verify the ROMs were built and installed correctly. If you've removed your power supply from the frame, the cord to the motherboard may just reach while your power supply is sitting on the edge of the chassis. For the first run, plug in your power supply, hold your finger on the ROMs, and press the power on button. If your Amiga doesn't power on (no green LED), immediately turn it off. If the ROMs get hot, immediately turn it off. If either happens, check that the sockets are installed correctly (for example, not off by a pin). If the Amiga does power on, verify that the power LED toggles from a dim state to a fully powered state. If it does, then you know that Kickstart has run at least far enough that you can confidently reassemble your Amiga.

Installing jumpers on the three headers will select the lowest address 256K block (each) of the two ROMs. Removing the jumpers will select the highest address 256k block of the two ROMs. This gives you access to a 512KB ROM image. If you've installed an enhancement such as Romy (by SpeedGeek) and run a wire from the A17 Bus header to the CPU A19, you will get access to a full 1MB image (ROM A16 pin is CPU A18). A much improved version of Romy can be found at https://github.com/reinauer/amiga-romy.

Enjoy!

More Details

Enter Romy Several years back (2011 or earlier), a very clever individual, SpeedGeek, decided that 512K is not enough. He developed some enhanced logic in a GAL chip that could be added to Amiga 3000 and 4000 computers to give them access to a larger ROM. This enhancement was named Romy, and it added decoding for a different address range, 0xe00000-0xe7ffff to give the CPU access to another 512KB of Kickstart ROM space. See https://eab.abime.net/showthread.php?t=61677 for more information. Newer versions of Romy now have experimental support for even large ROM sizes, up to 4MB, by decoding a larger ROM space. I won't explore the details of how that works. Just be aware that the CPU address lines A19, A20, and A21 can be made useful for accessing more ROM space in the Amiga 3000 and Amiga 4000.

A few examples of the possibilities opened up by the combination of Romy and the Amiga 3000 Rom Bank Switcher are illustrated below. I will combine the U180 and U181 EPROM tables to keep it all on the screen.

Imagine you want to have four different 1MB Kickstart images, which can be selected by switch or jumper settings. Here, the jumpers have been (arbitrarily) set to select Slot 1.

Q: How is the above configured using Romy and the ROM Bank Switcher?

• You will run a wire with address line A19 from Romy to the "Bus A17" pin on the ROM Bank Switcher.
• You will set and leave the A17/Bus jumper on the Rom Bank Switcher.
• You may run switches or set jumpers between the A18/Low and A19/Low headers to select which slot.

  Slot	A19	A18
  0	Low	Low
  1	Low	unset
  2	unset	Low
  3	unset	unset

Imagine instead that you want a single 4MB kickstart image. You'd first need a Romy configured to support 4MB mode. I believe Stefan Reinauer's Ultimate Romy supports this mode by default.

There is a single memory map in this case.

Q: Where do those ROM addresses come from?
The ROM address is simply a function of the CPU address bits 0-21, but then divided by two because the two ROMs make up the 32-bit wide value. [Remember at the top, we already chose to ignore the fact there are two bytes per ROM. If you want to speak in actual bus terms to the ROM, this would actually mean a further divide by two].
Thus, you can calculate the ROM address with:

ROMaddr = (CPUaddr & 0x380000) >> 1

(0x0e00000 & 0x38ffff) >> 1 = 0x00100000 (binary 00010000 00000000 00000000)
(0x0f80000 & 0x38ffff) >> 1 = 0x001c0000 (binary 00011100 00000000 00000000)
(0x1000000 & 0x38ffff) >> 1 = 0x00000000 (binary 00000000 00000000 00000000)

Q: How is the above configured using Romy and the ROM Bank Switcher?

• You will run a wire with address line A19 from Romy to the "Bus A17" pin on the ROM Bank Switcher.
• You will run a wire with address line A20 from Romy to the "Bus A18" pin on the ROM Bank Switcher.
• You will run a wire with address line A21 from Romy to the "Bus A19" pin on the ROM Bank Switcher.
• You will set and leave all three of the A17/Bus jumper and A18/Bus jumper and A19/Bus jumper on the Rom Bank Switcher.

Imagine that you want the option of two 2MB kickstart images. You'd need a Romy which is configured to support 4MB mode.

The possible configurations would be:

Q: How is the above configured using Romy and the ROM Bank Switcher?

• You will run a wire with address line A19 from Romy to the "Bus A17" pin on the ROM Bank Switcher.
• You will run a wire with address line A20 from Romy to the "Bus A18" pin on the ROM Bank Switcher.
• If you already have a wire with address line A21 from Romy to the "Bus A19" pin on the ROM Bank Switcher, it won't hurt anything to leave it there. It just won't add any value.
• You will set and leave the A17/Bus jumper and A18/Bus jumper on the Rom Bank Switcher.
• You will run a switch or connect a jumper to the A19/Low jumper. This allows you to select Slot 0 or Slot 1.

  Slot	A19
  0	Low
  1	unset

Q: Where do those ROM addresses come from?
Similar to the above, the ROM address is simply a function of the CPU address bits 0-20, but then divided by two because the two ROMs make up the 32-bit wide value. The A19/Low jumper ends up selecting bit 20 of the ROM address.
Thus, you can partially calculate the ROM address with:

ROMaddr = (CPUaddr & 0x380000) >> 1

and finish by inserting the bit value of A19/Low at bit 20

(0x0e00000 & 0x18ffff) >> 1 = 0x00000000 (binary 00000000 00000000 00000000)
(0x0f80000 & 0x18ffff) >> 1 = 0x000c0000 (binary 00001100 00000000 00000000)
(0x1000000 & 0x18ffff) >> 1 = 0x00000000 (binary 00000000 00000000 00000000)

(0x0e00000 & 0x18ffff) >> 1 | Bit20 = 0x00100000 (binary 00010000 00000000 00000000)
(0x0f80000 & 0x18ffff) >> 1 | Bit20 = 0x001c0000 (binary 00011100 00000000 00000000)
(0x1000000 & 0x18ffff) >> 1 | Bit20 = 0x00100000 (binary 00010000 00000000 00000000)