Table of Contents
Engineering Change for X688 Rev 0
Introduction
This engineering change adds a 74LS157, 2-input multiplexer, to swap chip selects of RAM and EPROM. This allows RAM to locate in address 0x0-0x7FFFF and puts EPROM to address 0x1000000 - 0x107FFFF. The 74LS157 is glued upside down in the “dead bug” fashion.
Design Files
Schematic of the original X688 rev 0 pc board
Schematic of the X688 after the engineering change is completed.
Procedures
Two cuts are made in the solder side of pc board as indicated by the two arrows. This two cuts disconnect RAM and EPROM chip selects. Click on the photo for high resolution image.
The 74LS157 is glued upside down in the orientation shown. Click on the photo for high resolution image.
The connections are as follow:
- 74LS157 pin 16, 14, 13, 11, 10 are connected to VCC
- 74LS157 pin 8 and 15 are connected to Ground
- 74LS157 pin 1 is connected to pin 12 of P90CE201 (A23/AP7)
- 74LS157 pin 2 and 6 are connected to pin 50 of P90CE201 (CSRAM)
- 74LS157 pin 3 and 5 are connected to pin 36 of P90CE201 (CSROM)
- 74LS157 pin 4 is connected to pin 22 of RAM (CS)
- 74LS157 pin 7 is connected to pin 22 of EPROM (CS)
