R89 SmartWriter 2 Rev. 89 (1991) (D. Clark) (3-EPROM)
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U21 - 2764 EOS $CE66
U22 - 27128 R89 (Upper 16K) $B97C
U8 - Not installed
U20 - 27128 R89 (Lower 16K) $86FB

Converting an older 4-EPROM configured console to 3-EPROM R89 configuration
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This all started when my first console had R77 with 4 EPROMs installed. After that, I had one with R80 with 4 EPROMs and another R80 with 3 PROMs. Some had a jumper wire bypassing the PCB A13 line and running the U22 A13 pin-26 directly to U7 (MIOC) pin-19 or also one end of unpopulated WJ5. This line appears to be used for special addressing to U22 for dealing with EOS in a 4-EPROM configuration. The schematic shows jumper settings but without any useful information about configurations.
 
Configuration jumpers for ROM select decoder (U23) and EPROM sizes used

Older 4-EPROM configuration:
WJ6 - Installed
WJ5 - Removed
U21 2764 (ALF-3 $4000 to $5FFF of full 32K ROM image)
U22 - 27128 (EOS in lower 8K and ALF-4 in upper 8K $6000 to 7FFF of full 32K ROM image)
U8 - 2764 (ALF-1 $0000 to 1FFF of full 32K ROM image)
U20 - 2764 (ALF-2 $2000 to 3FFF of full 32K ROM image)

Newer 3-EPROM configuration:
*Remove any prior wiring modifications to U22
WJ6 - Removed
WJ5 - Installed
U21 - 2764 (Dedicated to EOS)
U22 - 27128 (R89 Upper 16K $4000 to $7FFF of full 32K ROM image)
U8 - Not installed
U20 - 27128 (R89 Lower 16K $0000 to $3FFF of full 32K ROM image)

*Note a 27256 EPROM can be used to replace a 27128 EPROM as long as it is programmed to the upper 16K ($4000-$7FFF) space.


1/01/2021 by John Lundy