May 25, 2025

Sun 3/280

I had some questions I needed answered, so I pulled my 3/280 CPU board out of the chassis. This was a big of a challenge, as one of the extractor levers got broken off some time ago. The proper tool to remove the tiny screws is a 2mm allen wrench.
Just for the record, the part numbers on my boards are: 
501-1206 - sun3 280 cpu board
501-1576 - 16M ECC memory board

While the CPU board was out, I took some photos.

Click on the image below for a really big image of the board. What you will get is bigger than you may think (3840x3125), but will probably be resized to fit your browser.
You know what you need to do if you need or want to see it full size.

On the board

Like the Sun 3/160 there is no SCSI controller on the board, you were expected to add a VME card (actually probably a multibus card in a VME to multibus converter) to have SCSI. You do get ethernet via the notorious Intel ethernet chip.

Unlike the 3/160 with onboard ram, you do not get ram on the board! You are expected to provide ECC ram via an 8M, 16M or 32M ram card. So a minimum system with a 3/280 consists of two boards, the CPU and a memory.

From the hardware FAQ: 

        Processor(s):   68020 @ 25MHz, 68881 @ 20MHz (stock), Sun-3 MMU,
                        8 hardware contexts, 4 MIPS
        CPU:            501-1100/1206
        Chassis type:   rackmount
        Bus:            VME, 12 slot
        Memory:         64M (documented) physical with ECC, 256M virtual;
                        64K write-back cache, direct-mapped,
                        virtually-indexed and virtually-tagged, with
                        16-byte lines; 80ns cycle
        Notes:          Rackmount version of the 3/260.
                        Code-named "Sirius".

The board is 14.5 by 16.5 inches in size. This is one of the giant 9U VME boards that Sun was fond of in this era.

Some of the big items on the board are:

The connectors on the back panel are:

RAM gets displaced to add cache

You might be wondering (and grumbling) about no onboard RAM. Sun made two decisions. One was that these were "serious server boards" and would only be used with ECC memory. The other was that RAM would be evicted to make room for a cache.

Yes, unlike other sun3 machines, the 3/280 has an onboard cache. The bulk of it is made from 32 Cypress CY7C128-35 static ram chips Which are 2K by 8. So you use 16 of of these to give you a 32 bit wide word, and two banks of 16 would give you a total of 64K of cache.

This is in addition to the 256 bytes of I cache provided by the 68020. This instruction cache is set up as 64 words (words are 32 bits).

Feedback? Questions? Drop me a line!

Tom's Computer Info / tom@mmto.org