firmware board

The First Homemade Prototype

My Apple II Firmware Board Project

See my Brain Board page for the latest incarnation of the firmware board.

This board is a functional recreation of an original Apple II firmware board that uses inexpensive 27c128 or 27c256 EPROMs.  27128  or 27256 EPROMs should also work fine.  Because of the denser EPROMs used, 6 sockets are replaced with one and the resulting size of the board is reduced to approximately 2 by 3 inches.

The board supports all functionality of the original Apple board including:

In addition, I have added several enhancements in order to maximize usefulness of this board.  The modified design will allow booting a ROM-less motherboard into DOS, which is not possible with the original firmware board.  The neat thing about these enhancements is that they didn't require any additional chips.  The board already had a spare flip-flop available on 74LS74 and just required adding some jumpers and minor rewiring.

When in motherboard disable mode, and with a 27c256 PROM installed, you can even switch between the two banks of 27c256 PROM memory either with the switch or through software control, as if you had a ROM on the motherboard and an original Apple firmward card installed.    

Here is a description of the 4 jumpers used to configure this board.

  1. Motherboard ROM enable/disable - completely shuts down the motherboard ROM during memory access to EPROM address space D000-FFFF or D000-F7FF if ROM F8 is disabled on the FW card.  Uses apple's inhibit line for this purpose. 
  2. ROM F8 enable/disable - disables or enables access to memory bank F800 on firmward card. If disabled, F8 ROM accesss are handled by the motherboard.
  3. High bank force/program -  If configured to force - all accesses to firmware card 27c256 will address high half of this card.  If set to program, high bank can be selected if force on low bank is not set.  The on board flip-flop is set to select high bank either by the switch on power up or by software setting the switch.  This flip-flop works the same way as the original switch on the firmware card, but can be used to select between high and low banks of a 27c256 when the motherboard ROM is disabled.
  4. Low bank force/program -  If configured to force - all accesses to firmware card 27c256 will address low half of this card.  If set to program, low bank can be selected if force on high bank is not set.  The on board flip-flop is set to select low bank either by the switch on power up or by software setting the switch.

Here is a table showing the valid configurations of the 4 jumpers.  

Motherboard ROM disabledMotherboard ROM enabled
High Bank 27c256 ForceHigh Bank 27c256 ProgramHigh Bank 27c256 ForceHigh Bank 27c256 Program
F8 enableLow Bank 27c256
Force		invalid settingLow FW board 
27c256 bank
uses low 27c246 F8 bank		invalid settingswitchable between
motherboard and FW board
low 27c256 bank
switch or software selectible
uses currently active F8 bank
Low Bank 27c256
Program		High FW board 
27c256 bank
uses high 27c246 F8 bank		switchable between
low or high
FW board 27c256 bank
switch or software selectible
uses currently active F8 bank		switchable between
motherboard and FW board
high 27c256 bank
switch or software selectible
uses currently active F8 bank		invalid setting
F8 disableLow Bank 27c256
Force		invalid settinginvalid settinginvalid settingswitchable between
motherboard and FW board
low 27c256 bank
uses motherboard F8 ROM
Low Bank 27c256
Program		invalid settinginvalid settingswitchable between
motherboard and FW board
high 27c256 bank
uses motherboard F8 ROM		invalid setting

A schematic with my modifications can be found here.

If you are interested in purchasing a kit or an assembled board, take a look at the Brain Board page.

Schematics for the original Apple Firmware board and a description of operation can be found in the book "Understanding the Apple II", which is available online in a couple of places. When using 27c128 PROMS, the jumper settings must be set so as to force the address to select high bank.

27c128  or 27c256 parts are programmable using inexpensive Willem prom programmers that are very inexpensive on EBAY.  I bought my programmer on EBAY for only $19.00 on ebay.  Parts come with or without windows which make them erasable or not.  I bought several without windows since they are very inexpensive (around $1.00 each) and I didn't want to bother with finding or building an EPROM eraser.

Other than the enhancements listed above, the design is pretty much the same as the Apple Firmware board design.  I tried to find a way to reduce chip count, but couldn't find a way to do it, without resorting to programmable logic.  The major difference is that the individual chip selects coming out of the 74LS138 are not used.  Instead the 74LS244 enable coming from the 75LS11 is also used to select the EPROM.  Additional address lines are routed to the EPROM in order to support the extra address space in these parts.  These address lines were already connected to the 74LS138.  The output of the 74LS74 flip-flop which enables the Firmward card via the 74LS138 is also used to enable the EPROM.  

One thing to note is the gaps in addressability when using the 27c128 or 27c256 EPROMs on this board which can only addresss 12K of memory.  Use the table below in order to program the EPROMs correctly.

Apple ROM Address Space 27c128 or 27128 Address
(high bank selected)		 27256 or 27c256 Address
(high bank selected)		 27256 or 27c256 Address
(low bank selected)
not addressable 0000-0FFF 4000-4FFF 0000-0FFF
D000-D7FF 1000-17FF 5000-57FF 1000-17FF
D800-DFFF 1800-1FFF 5800-5FFF 1800-1FFF
E000-E7FF 2000-27FF 6000-67FF 2000-27FF
E800-EFFF 2800-2FFF 6800-6FFF 2800-2FFF
F000-E7FF 3000-37FF 7000-77FF 3000-37FF
F800-FFFF 3800-3FFF 7800-7FFF 3800-3FFF


The layout has chips flipped upside down verses typical Apple II expansion cards in order to simplify layout.  It turns out that I was able to layout this board with only a few vias.

Here is the parts list, with Mouser parts numbers for many of the parts.  Total cost (minus the PCB) should be less than $20.00.

PART QUANITTY MOUSER PART NUMBER
74LS09 1 595-SN74LS09N
74LS11 1 595-SN74LS11N
74LS74 1 595-SN74LS74AN
74LS138 1 595-SN74LS138N
74LS244 1 595-SN74LS244N
27C256  1 511-M27C256B-90B6 (OTP - cost around $1.00)
jumper 4 538-15-29-1025
right angle switch 1 633-M2022S2A2W30-RO (most expensive item - around $6.00)
molex KK 1 538-22-10-2021
3K resistor 4 CF1/4CT52R302J
10K resistor 2 CF1/4CT52R103J
.1uF +80%/-20%  capacitor 5
16 pin socket 1
14 pin socket 3
28 pin socket 1
20 pin socket 1

Here is a view of both layers (red is top, blue is bottom) of the layout with silkscreen (black).

  apple II firmware board layout

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