STEbus 68000

68000 processor board renovation

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Renovation of a tried and tested commercial design. Not my own design!

This was not a best seller.

I suspect the PAL chips are a lot more complex than the 68008 or 68020 designs. They must interface the CPU's 16-bit data bus to the 8-bit STEbus. There is a 74LS646 instead of the usual 74LS245 bus data buffer.

I saw one for $995 on eBay which was way beyond my budget. I recently saw one for $220, still not trivial but I bought it.

From the ASCII text in the firmware, I believe this board came from an OS-9/68000 system made by VG Elemental, and a product called the PlasmaQuad.

VG Elemental seem to have been absorbed by Thermo Scientific.


Logic equations for data bus interface manager.

plain - 3.92 kB - 03/29/2022 at 15:16



Logic equations for interrupt priority encoder.

plain - 1.94 kB - 03/29/2022 at 15:16



Logic equations for RAM and ROM address decoder.

plain - 2.27 kB - 03/29/2022 at 15:16



Logic equations for larg-scale address decoder.

plain - 3.30 kB - 03/29/2022 at 15:16



Logic equations for bus arbiter.

plain - 2.20 kB - 03/29/2022 at 15:16


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  • Programmable logic

    Keith03/25/2022 at 16:54 0 comments

    I've got a bit further analysing the equations than the files uploaded on the 13th March, but for some reason I am unable to upload files as of now.

    The 68008 board PAL chips provide good hints about the logic. 

    The 68000 logic seems to be complicated by the fact the data bus interface is neither 8-bit (like the 68008) nor can it be made so by dynamic bus sizing (like the 68020).

    Instead, the logic appears to generate two 8-bit STEbus cycles for every 16-bit CPU cycle.

    One of the bytes is temporarily held in a latch (a 74ACT646) while the other passes through a buffer (a 74ACT245). These present two bus loads that are in total probably less than one LS TTL load.

  • Sourcing

    Keith02/19/2022 at 21:52 0 comments


    Spotted one for $220. Bought it. Works out to be $327.22 or £240.77 after import duty and postage. Not cheap but STEbus evangelism is my bag. Due to arrive in March, when I shall read its programmables!

  • About the PlasmaQuad 3

    Keith02/25/2021 at 13:35 0 comments

    According to

    The new VG PlasmaQuad 3 system is the "highest performance quadrupole ICPMS ever built, and the most automated" claims the company.  It introduces a range of new features, including a fully automated dual-mode plasma torch designed to operate in the standard mode for the majority of elemental determinations, and in a cool plasma mode for the determination of selected elements; the "S" option interface to dramatically increase sensitivity allowing routine determination of sub ppq levels; Super-Quant to automatically provide accurate, interference-corrected concentrations; and, AutoRange plus, an advanced detector system providing a linear dynamic range of eight orders of magnitude, increases accuracy and productivity.

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