Slow fail-safe logic

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LSL-IC FZH251, manufactured in early 1987

Slow fail-safe logic (LSL, English low-speed logic ), also known as high-level logic (HLL) or high threshold logic (HTL), is an outdated electronic circuit technology ( logic family ) for logic circuits ( gates ).

technology

LSL circuits were developed from DTL circuits . By increasing the supply voltage to 15 volts, the interference immunity was increased by a higher signal spacing between the low level and high level , but other properties, such as the signal speed, take a back seat. The input switching threshold is raised to approx. 6 volts with the aid of a Zener diode .

LSL circuits are relatively slow (signal transit time approx. 200 ns) and were mainly used in the 1970s and 1980s. A typical circuit of this logic is the SAA1029 or the type D410D copied by the Frankfurt (Oder) semiconductor plant . The slow fail-safe logic is practically hardly used today and has lost its meaning.

construction

Structure of a NAND gate in LSL technology
Type: FZH101A; P V  = 180 mW; t pd  = 175 ns

The illustration opposite shows a dimensioned LSL-NAND gate with an operating voltage of 12 V, which occurs more frequently than 15 V in real applications and a signal-to-noise ratio of around 5 V. To increase the output load capacity ( fan-out ), a push-pull output stage is added intended. By using slow transistors and a capacitor that can be connected externally , the dynamic interference immunity can be increased even further, as the capacitor only recharges slowly and thus suppresses short-term interference voltages, even if these interference voltages are greater than the intended signal-to-noise ratio. However, this increases the switching times considerably.

Related logic families

Related precursor logic families of the slow fail-safe logic were the resistor-transistor logic (RTL) and the diode-transistor logic (DTL). These two logic families are out of date and are practically no longer used today. The structure of the slow fail-safe logic is similar to that of the transistor-transistor logic (TTL). The TTL family has numerous subfamilies and is widespread today.

Another logic family to be mentioned is the emitter-coupled logic (ECL), in which the use of this technology has been declining today.

There is also the complementary MOS logic (CMOS). This family of logic is currently very common.