One of the key moments in the composition of the conditions of possibility for a digital abstraction, within which certain logical operations might be combined, performed and arranged to carry out algorithmic computation took place in 1961 when James Buie who was employed by Pacific Semiconductor patented the Transistor Transistor Logic (TTL). This was an all transistor logic for analogue circuitry that crucially standardised the voltage configuration for digital circuitry (0v-5v). This represented a development from the earlier Diode–transistor logic (DTL) which used a diode network and an amplifying function performed by a transistor, and the even earlier Resistor–transistor logic (RTL) based on resistors which handled the input network and bipolar junction transistors (BJTs) as the switching devices. The key to these logic circuits was the creation of a representation of logic functions through the arrangement of the circuitry such that key boolean logic operations could be performed. TTL offered an immediate speed increase as the transition over a diode input is slower than using a transistor. With the creation of the TTL circuitry the logical operations of NAND and NOR allowed the modular construction of a number of boolean operations that themselves served as the components of microprocessor modules, such as the Adder.
I want to explore the importance of signal in relation to the interface between the underlying analogue carrier of the digital circuitry and the logical abstraction of digital computation – that is the maximisation of signal over noise in the creation of a digital signal carrier. It is exactly at this point that the emergence of digital computation is made possible, but also a suggestive link between signal/noise that points to the use of abstraction to minimise noise throughout the design of the digital computer, and which creates a logical universe within which computational thinking, that is signal without noise, or without noise as previously understood as thermal noise, is a constituent of programming practice. This is useful for developing an understanding between notions of materiality in theorising the digital, but also in making explicit the connection between digital “signal” and voltage “signal” or between the possibility of communication of information in a digital system.
At its most basic level standard TTL circuits require a 5-volt power supply which provides the framework within which a binary dichotomy is constructed to represent the true (1) and the false (0). The TTL signal is considered “low”, that is “false” or “0”, when the voltage is between the values of 0V and 0.8V (with respect to ground) and “high”, that is “true” or “1” when the voltage lies between 2.2V and 5V (called VCC to indicate that the top voltage is provided by the power supply, known as the positive supply voltage). Voltage which lies between 0.8V and 2.0V is considered “uncertain” or “illegitimate” and may resolve to either side of the binary division depending on the prior state of the circuitry or be filtered out by the use of additional circuitry. The range of voltages allows for manufacturing tolerances and instabilities of the material carrier, such that noise, uncertainty and glitches can be tolerated. This tripartite division creates the following diagram:
|Tripartite division of voltage in TTL digital circuitry|
This standardisation of the grammatisation of voltage creates the first and significant “cut” of the analogue world and one which was hugely important historically. By standardising the division of the binary elements of digital computation, in effect, the interoperability of off-the-shelf digital circuits becomes possible, and thus instead of thinking in terms of electrical compatibility, voltage and so forth, the materiality of the binary circuit is abstracted away. This makes possible the design and construction of a number of key circuits which can be combined in innovative ways. It is crucial to recognise that from this point, the actual voltage of the circuits themselves vanishes into the background of computer design as the key issue becomes the creation of combination of logical circuits and the issues of propagation, cross-talk and noise emerge at the different level. In effect, the signal/noise problematic is raised to a new and different level.