27C16 EPROMs

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27C16 EPROMs

I finally made some progress on EPROMs for an Apple II Replica kit. What I found and suspected was the simple pin 18 and pin 21 swap shown in various websites and in the Understanding the Apple II book doesn't work with a language card. The language card pulls down pin 18 (INH_L) in the Apple II design (which becomes pin 21 once swapped). Since pin 21 is VPP on a 27C16, it can't be pulled down by the language card.

I have an Apple II+ that had a bad PROM. What I did was use a 27C32 EPROM and dead bugged an 74LS04 inverter for pin 18. Since pin 21 on that device is an address pin it was OK to leave it tied to pin 20.

The 27C16 is a little more complicated. The adapter I came up with involves clipping pins 18 and 21 on the EPROM so they don't short out against the socket. I used a Tiny Logic inverter (NC7S04). The wiring which is done underneath the EPROM is as follows:

27C16 pin 24 to NC7S04 pin 5 to 27C16 pin 21 (VCC)
27C16 pin 12 to NC7S04 pin 3 (GND)
27C16 pin 18 to NC7S04 pin 4 (CS_L)
Socket pin 18 to NC7S04 pin 2 (CS)

For my first attempt I ordered SC70 packages. They are too small to work with. I then ordered SOT23s which are more manageable. I did a test run by replacing the II+ 27C32 with a 27C16 with the above rework. It works fine. I did use a low profile machine pinned socket between the motherboard and 27C16. It is really not necessary. And once you plug a machined pin socket into a regular socket, don't expect the regular socket to work with anything but a machined pin.