XE 1M Cart Instructions (Rev A)

by Bryan Edewaard

This board is designed to fit into the new Mq/Sikor shell and provide up to 1MB of ROM space via bank-switching. It can also be configured for a non bank-switched 16K ROM.

The switching scheme is based on the XE cartridges produced by Atari. Any access within 8000-9FFF will read from the selected 8K bank, and any access within A000-BFFF will read from a fixed bank (the topmost 8K in the ROM). A write to the D5 page (D500-D5FF) sets the current bank.

Theory of operation:

U2 is a 374 8-bit register that holds the selected bank. While accessing A000-BFFF this chip is disabled, and the installed resistors (R1,R2,R3 or RN1) pull all the bank-switching address lines high causing the last bank to be read. While accessing 8000-9FFF the 374 is enabled and the selected bank is read.

U3 is the 28-pin footprint for a 27128-27512. U4 is the 32-pin footprint for a 27010-27080. U1 is a 00 quad NAND gate (combinational logic), and C1 & C2 are .1uF decoupling caps (for noise suppression).

R1, R2, and R3 are the pull-ups for A13, A14, and A15 respectively. In their place RN1 (SIP resistor network) can be used to pull up lines A13-A19

I typically use 74HCT logic with this board, but just about any TTL logic (such as LS, ALS, F, etc.) should work.

Here are the specifics for using different size EPROMs:

-16K (27128):

Resistors: Omit R1, R2 & R3 = jumpers

The bankswitching is disabled for this size and U2 is omitted (install U1, however). In the space for U2, install a jumper between pins 1 & 2. Also install R2 & R3.

-32K (27256):

Resistors: R1 & R2 = 4.7K, R3 = 1K, omit RN1

Set bits 0 and 1 to the desired bank.

With this size EPROM we need to address the problem of the VPP pin (pin 1). This pin is used for A15 in larger EPROMs, so the cartridge is designed to be able to toggle it. But most 27256 EPROMs will stop functioning if this line goes low. Since the 374 is in an unknown state at power-up, we need a way to make sure the EPROM will always respond. There are a couple possible solutions to this problem:

1. The easy method: Bend up (or cut off) pin 6 of the 374 so R3 always keeps VPP pulled high. This method will work regardless of the ROM image (when using this method you can replace R3 with a jumper wire if you wish).

2. The software solution: Set the diagnostic flag in the cartridge. This way, the OS will only check one byte at the end of the cartridge before jumping in so the 374 is never engaged before you get a chance to set it. From then on, always keep bank-select bit 2 set to keep VPP high. Remember that a diagnostic cartridge must initialize the system itself, so you cannot use this option unless you specifically write for it. R3 is 1K to make sure it meets the current requirements for the VPP pin.

-64K (27512)

Resistors: R1, R2 & R3 = 4.7K, omit RN1

No surprises here. Just set bits 0,1 and 2 to the desired bank.

-128K (27010)

Resistors: Omit R1,R2,R3 and install an 8-pin bussed 4.7K network in RN1.

Set bits 0,1,2 and 3 to the desired bank.

-256K-1MB (27020-27080)

Same as above, but add one more bank-select bit for each increase in EPROM size (bit 7 is never used).

Notes:

Always remember that the IC's face the back of an Atari 8-bit computer cartridge.

C1 & C2 can be any small, inexpensive .1uF (usually marked '104') ceramic or monolithic cap.

The board is designed for 1/4 watt resistors. However, 1/8 watt will work too.