FAQ
Short answers to the questions that come up most often with the RAM expansion. The details are in Address jumpers and memory map and How it works.
The banner still shows 1048576 bytes. Why?
The card is not seen. In order of likelihood:
- JP4 on the main board is open. It must be shorted with this card: the main board then answers the CPU for the card. This is the most common cause.
- JP1–JP3 on the card are not all open. For one card they must be.
- IC1 is turned round, not pressed fully into its socket, or not programmed with the card's decoder.
- A joint on J1 is missing or cold. Look at all 64.
- U1 or U2 is turned round or has a folded leg: the first megabyte of the card must work for anything to be counted.
Why does JP4 on the main board matter?
Every access by the 68010 waits for an answer called DTACK. This card does not send one: its memory is fast enough that the main board can answer for it. Shorting JP4 tells the main board to do that for all of expansion space. With JP4 open, nobody answers and the access ends in a bus error.
The banner shows 2, 3 or 4 MB instead of 5. What now?
The count stopped inside the card, at the pair of chips that failed:
- 2097152: U3 or U4.
- 3145728: U5 or U6.
- 4194304: U7 or U8.
Reseat that pair, look for a folded leg, and check the notches. If that does not help, swap the pair with U1 and U2: if the fault moves, it is the chip; if not, look at the joints of those sockets.
Do I need to change anything in the software?
No. The firmware finds the memory at start-up and reports it in the banner. Programs get the larger total from the firmware.
Can I use several cards?
Yes, up to four, with a bus board: the main board has room for only one. Set them to 1, 5, 9 and 13 MB so they follow each other without overlapping. All of r2 expansion space is then filled: 14 MB in total, with only 1 MB of the fourth card in use. JP4 on the main board stays shorted.
Two cards must never share a base address.
I set the card to 5 MB and the banner shows 1 MB. Is it broken?
Probably not. The firmware counts only memory that continues without a gap from the main board's. A card at 5 MB on its own leaves a hole from 1 to 5 MB, so it is not counted, even though it works. For one card, leave JP1–JP3 open: base 1 MB.
Why only 3 MB at base 11 MB, and 1 MB at 13 MB?
On the r2 main board, expansion space ends at $DFFFFF: from $E00000 up is ROM and I/O. A card set high enough reaches past that line, and the part above it is never selected.
Can I fit fewer than eight chips?
Yes. Fit whole pairs starting with U1 and U2: 2 chips give 1 MB, 4 give 2 MB, 6 give 3 MB. Leave no empty pair in between, or the count stops there.
Does the card work in the bus board?
Yes, in any slot, the same way it goes on the main board: component side down, with the beep test before power-on. JP4 on the main board must still be shorted. The bus board is what lets you run more than one card.
Can I use it with other expansion cards?
Yes. Cards in I/O space, such as video or serial cards, are not affected. A card that also uses expansion space must not overlap the RAM card's addresses, and must work without extra wait time, because with JP4 shorted the main board answers every access there straight away.
IC1 is blank, or I lost it. Can I program a new one?
Yes. IC1 is an ATF16V8 programmed with the card's decoder logic. The JED file is in the rosco_m68k hardware projects repository, under ram-expansion/code/pld. With a TL866II+ programmer and minipro, the included burn.sh writes it.
Is it safe to fit or remove the card with the power on?
No. Always switch off first. Fitting it live, or turned the wrong way round, can destroy chips on both the card and the main board.
Still stuck?
Send clear photos of both sides of the card, the jumpers on the main board and the full start-up banner to Telegram or solderdemon.official@gmail.com.