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IBM SLT

1960s IBM Mainframe Technology Lives!

IBM SLT

After completing the MK14 reproduction project featured on this website, I got curious as to where the core concepts of the microprocessors of the 1970s had come from. After reading the excellent IBM System/360 by Pugh/Johnson/Palmer it turns out that a lot of them came from the IBM System/360 project of the 1960s

This $6bn "bet the company" effort transformed computers from being a collection of discrete transistors on cards, each with their own individual acrchitecture and instruction set, into a line-up of (relatively) compact hybrid based machines with a >20:1 performance range. Key concepts such as microcode, the 8 bit byte and backwards compatibility of code (reference the x86 architecture) can trace their roots back here.

The book includers coverage of the core component technology used to reduce the size, increase the performance, and transform the reliability of these new generation machines - SLT. Solid Logic Technology modules were hybrid circuits, combining diced transistors and diodes with screen printed resistors to integrate a logic gate into a single 0.5" square can. SLT modules were built into cards, cards into boards, boards into (fold-out) gates, and gates into complete computers

As the first step on the long road of shrinking the size of digital logic, this new technology was a pivotal point in the evolution of computing, and got me thinking what could be made today with those devices. Clearly, constructing a whole computer would be impossible due to size, component availability, and the lack of the engineering resource that IBM deployed on the original development! (And it's been done...) However, something smaller, with a few hundred gates would be feasible.

Something that fits this description well, and that is also a classic "retro" project would be an SLT powered Nixie clock. Using a conventional Johnson counter architecture, an HH:MM:SS clock could be made from a few hundred SLT devices. SLT modules are found on ebay from time to time, and an initial purchase let me check the datasheets I have for some of them, and my understanding of how to use SLT to make digital logic. The photo below shows this work in progress, with one SLT mounted on a breadboard, using a specially made 4x4 1/8" grid (SLT pinout) to regular 0.1" DIL converter.

Checking parts

SLT Design Prototyping.

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The completed clock will use SLT logic, Russian Nixie tubes, and some discrete power transistors to drive the display. The finished result shoud look something like this:

SLT Nixie Clock Concept

SLT Clock Concept.

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This is still work in progress. Basic NAND gate operation (AND-INVERT in 1960s IBM parlance) has been checked. Next stage is to build a 2 or 3 bit counter to verify the level of abstraction up. I am in need of both a further supply of SLT modules, and more information on some that I have that are not listed in the publically available documentation (IBM clamped down on information release after 360 clones started to appear). A pinout and circuit equivalent is needed for the following devices:

Please email me if you could help with any of these!

One minor contribution to current knowledge of SLT - I couldn't find a package pin-out anywhere for these devices. A little investigation of the parts I have has revealed the following (viewed from above, including pins 13-16 as used on later generation SLD, ASLT and MST devices).

SLT/SLD/ASLT/MST pinout

SLT/SLD/ASLT/MST Pinout.

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