A Modified SB180, 22MHz
I have modified a stock SB180 to run at 22Mhz. I removed all of the DRAM chips, DRAM address
multiplexers, and the EPROM, and attached a custom made circuit board with 256kb of static RAM
and a 32kb EPROM.
The memory board was designed using Eagle CAD
I am a cheap bastard, I used the Linux Freeware version and so had to cram all of the desired
chips onto the Freeware's board size limit of 100 x 80 mm, with only two signal layers. Here
are the CAD files for the board
, and the project file
Notes for the schematic:
* All of the signals enter the memory board through IC1, IC5, IC6, and IC7. These sockets also provide the +5VDC power and ground from the SB180 board.
* IC1 is the EPROM socket on the SB180 board (U16). This provides the data bus (d0-d7), the addresses a0 through a14, the OE signal, the CE signal, and VPP.
* IC2 is the EPROM socket on the memory board. I use an STI M27C256B-70 32kx8 70 nsec EPROM.
* IC3 and IC4 are the static RAM sockets on the memory board. I use two Toshiba TC551001APL-85 128kx8 85 nsec static RAM chips.
* IC5 is the socket for DRAM chip U8 on the SB180 Board. This socket provides the WE signal.
* IC6 and IC7 are the sockets for the memory address multiplexers U10 and U12 on the SB180 Board. These sockets provides addresses a15, a16, and a17.
* IC8 is a 74x04 hex invertor chip, of which only one invertor is used. I used a 74ACT04.
Notes for the SB180 board:
* I removed all of the DRAM chips, the three address multiplexers (U10, U11, and U12), and the wait-state chip U15.
* I replaced chips U10, U14, U21, U22, U23, U25, and U26 with 74ACT parts.
* I installed a 22.1184 MHz crystal, and a Zilog Z8S18020PSC Z180 processor.
* I changed jumper JP1
from the 8/16K setting to the 32K setting.
Notes on software:
* The EPROM initially configures the processor to run at 11.0592 MHz. This clock speed allows the console serial port to run at 115.2k baud.
* The EPROM code is a modified version of the stock SB180 EPROM code. I made changes to accomodate the higher clock speed, serial port baud rate choices, a RAM memory test, and some other bits and pieces. This code is available via email.
* I then boot into B/P-BIOS
, where the switch to 22.1184 MHz operation is done.
Click on a thumbnail-image for a larger image.
|SB180, 22MHz, with static RAM memory board
|SB180, 22MHz, with memory board removed
|SB180, 22MHz, side view, showing stack
|SB180 Memory Board CAD Image
|SB180 Memory Board Schematic Image