4164/41256 RAM Modification

Williams MPUs shipped with 24 × Mostek 4116 DRAM (16,384 × 1-bit), introduced in 1977 — only a few years before Defender.

4116 pinout

These chips need three voltages (triple-rail):

• +5 V — TTL input/output buffers
• +12 V — primary power
• −5 V — substrate bias / shield

If −5 V drops while +5 and +12 are still present, the chip overheats and fails quickly.

That triple-rail requirement made the 4116 a weak link on these boards, especially as the linear supplies aged and the rails got noisy.

4164 First Successor

In the early 1980s the direct successor appeared: the 4164 (65,536 × 1-bit).

An internal bias generator supplies the −5 V, so the part runs from a single +5 V rail. Pins that used to carry the extra supplies became another address bit, which is how capacity went up. 4164s generally run cooler and tolerate supply variation better.

41256 Last 16-Pin Successor

By the late 1980s density had increased again. Using the last unused supply pin as another address line gave the 41256 (262,144 × 1-bit).

Pinout Comparison

Here is a comparison in pinout:

4116 vs 4164 vs 41256 pinout

Modify the MPU

What we need to do: For the 4164/41256 to work, we need to accomplish a few things.

• Remove the -5v from Pin 1
• Remove the +12v from Pin 8
• Substitute +5v on Pins 1 and 8 (If we left these unconnected/floating, the address value would randomly change
• The goal is to have +5 on Pins: 1,8 and 9. Ground stays on Pin 16

Original MPU Power Connector Pinout

Original MPU Power Connector Pinout

Step 1 — Cut pins 5(+12 Regulated) and 9(-5v) * Keep one of the cut off pins *

• Note - The 'Polarizing Key is on Pin 4. With the mod, if the connector is reversed/flipped it can't be plugged in *
• Note - Pin 1 is on the right and Pin 9 is on the left *

Pins 5 and 9 cut

See also

• Hardware - General
• Battery Damage