Logo SB180
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. Since 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, the schematic, 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

810x441, 80k

SB180, 22MHz, with memory board removed

809x439, 95k

SB180, 22MHz, side view, showing stack

601x283, 23k

SB180 Memory Board CAD Image

1140x945, 35k

SB180 Memory Board Schematic Image

2946x2249, 70k


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