A small bit of technology history is necessary to understand the reasons that IBM created SLT. These lead to the peculiarities that made my project more daunting than I had assumed when I began this quest a year ago. At the time that IBM was developing SLT for the upcoming "bet the company" S/360 launch, the industry was just transitioning from discrete transistors to integrated circuits in 1958, after the invention of the IC by Kilby at TI and by Noyce at Fairchild. For the first few years of the 1960s, ICs were very expensive and primarily used in aerospace and other defense areas where weight or other characteristics of the IC were all important. The industry was still debating the best way to build logic gates, with adherents promoting schemes such as DTL (diode oriented with transistors as active switches), TTL (transistor-transistor), and RTL (resistor-transistor). Traces of all these approaches are visible within SLT, which tends to the use of diodes and transistors in many of its building blocks. These were legitimate debates because the tradeoffs of speed, cost, size and power requirements were important choices when a large computer would fill a room just by itself. The eventual economics of ICs weren't obvious, as the industry was delivering small volumes at very high prices to demanding military specs that yielded many rejected chips during manufacture.
However, the cost advantages were obvious from reducing the number of discrete components on a board and the amount of wiring that had to occur between these parts. In addition, the circuits could operate faster because of shorter distances between the devices and other characteristics that reduced delays. IBM had to make some important choices then gamble on these by ramping up an entire supply chain from silicon to delivered computers that were based on the approach they would choose.
Had the choices been made just two or three years later, the core of SLT would undoubtedly have been different and based on integrated circuits. What IBM chose to base their entire next generation on was continued use of germanium transistors, diode oriented gates but to place the transistors and diodes on a ceramic substrate in little square modules.
|SLT ceramic module built up with transistors, diodes and resistors|
|Typical SLT card|
The SLT modules were portions of a logic gate or function, completed with external components or requiring more than one of the modules to comprise a single logic function such as a flip-flop. The basic module used most often in SLT has an AND gate, an OR gate and an inverter (NOT gate) inside in an arrangement they called AOI. Three diodes were wired together to form the AND function, that was joined to another diode that provide the OR function, and those four wired together diodes connect to a single transistor that is the inverter.
|IBM's AOI module|
The flip flop used in many places in an 1130 or 360 computer required two AI (AND -Inverter) modules, a resistor pack (several resistors molded into one long component, and an RC pack (a mix of resistors and capacitors molded into one component).
Reading the IBM logic diagrams (called Automated Logic Diagrams or ALDs) requires you to understand how this technology works and how it is combined in modules on cards to make up logical functions that a modern digital designer would recognize.
This becomes much more difficult because IBM terminology at that time does not match the way that modern digital engineers refer to technology. IBM will refer to DC set or DC reset of a flip flop, or say that it is AC triggered (showing the connection on the ALD with a capacitor symbol just in front of the input), or refer to 'binary' triggering.
The IBM flip flop is not any of the well understood flip flop types a digital designer would recognize - not D, SR, or JK or other variants - and it has behaviors that are decidedly odd. If a flip flop is already in the 'on' state and a signal is applied to switch it on, the flip flop will produce a short pulse or signal from the opposite (off) output. This would wreak havoc with most designs if the engineer didn't expect it, as this would be a defect that would not be accepted from a modern logic component.
|IBM Description of Flip Flop operation|
I had to do quite a bit of research and really understand what was going on, down inside the analog components in the SLT modules as well as in the digital designs shown in the SLTs. In order to do that, I would need to understand electronics and electrical engineering to a much deeper level than I had ever attained as a hobbyist/hacker/bludgeoner of electrical things. This leads to the next post - my pursuit of enough EE knowledge to move forward with the 1130 project.