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TTL logic

Logic chips from 74 series. Simple gates, counters, buffers, multiplexers, decoders, latches and other interesting functions. They are povered from +5V supply. Low state means below 0.8V, high state - over 2.0V. Voltage range 0.8V to 2.0V is prohibited and may give uncertain results.

The TTL chips are available in vide range of technology: S, F, LS, L, LV, LVX, HC, HCT, HCU, ALS, AC, ACL, ACT... The LS is used below as the more popular.

In new designs use LS (or ALS); use HCT when low power consumption is critical and HC only when you cannot get the LS/HCT version. Not recomended are S, F, and "no letter" except when the high output current or speed are critical. The LV and LVX are dedicated to lower power supply voltage (1V to 3.6V).

Simple gates

Simple gates are used for the basic logic operations like AND (74LS08), OR (74LS32), NOT (74LS04, 74LS06), NAND (74LS00, 74LS132, 74LS10, 74LS20, 74LS30), NOR (74LS02, 74LS27), XOR (74LS86).


Counters are very rare used in 8bit computers, mostly in clock cycle signal generators (74LS393), video generators, DMA circuits and mouse interfaces.

Up/down counters can be used as mouse interfaces. Most schemes use 74LS193 (4bit), but ideal chip is 74LS469 (8bit, 3-state outputs).

Buffers and amplifiers

Buffers are quite often used. The 74LS07 (open collector) amplifier is used mostly in FDC circuits (to drive the flat cable, together with 74LS06 or 74LS38).

The mostly used tri-state buffer is 74LS245 (74LS645). The 74LS244 is not recomended (except in specjal cases). Quite often are used four separate tri-state buffers - 74LS125. Interesting are 6bit buffers: 74LS365, 74LS367 and their inverter versions: 74LS366, 74LS368. Useful may be also 3-state version of multiplexers: 74LS257, 74LS253, 74LS251.


The 74LS157 is most popular in DRAM circuits, for address lines multiplexation. Sometimes 74LS153 is used, especially when DRAM is accessed by two devices (like Z80 and CRTC in Amstrad CPC).

But multiplexers are also very useful in simplifing advanced logic schemes. The 74LS157 selects one of 2 sources for 4 lines, the 74LS153 - 4 sources for 2 lines, the 74LS151 - 8 sources for one line. Their gate input are usually useless, as force low state on outputs. So, sometimes better are their tri-state output versions, where gate input can make a useful function: 74LS257, 74LS253, 74LS251 (however, they are harder to get).

Caution! The 74*157 (at least some types of them) can produce hazard. 20ns low state appears on the output when both inputs are high and selection changes from 0 to 1. This has no matter when driving memories, but driving strobe signals can be a big problem. Hazard chips: SN74ALS157N PORTUGAL 8817XF, T74LS157B1 W991A9322 MALAYSIA. Non-hazard: GSS GD74LS157 F 02, GS 9439 GD74LS157.


Decoders are used to distinguish I/O port addresses and memory locations (one particular address or group of addresses).

The most popular decoders are 74LS138 (6 inputs to 8 outputs) and 74LS139 (3 inputs to 4 outputs - two times). A powerful decoder is also the 74LS688 (11 inputs to 1 output).


Simple RS (reset/set) latch can be made of two NAND or NOR gates. But their usage is very limited - like first write detector or button noise filter.

The 74LS74 has two 1bit latches with full amount of lines: data input, clock (rising edge), reset, set, output, inverted output. It is most universal, however there are only 2 bits in one case.

Larger latches are 74LS174 (4 bits; with negated outputs, too), 74LS175 (6 bits, reset feature), 74LS273 (8 bits, reset feature), 74LS573 (8bit, transparency - can work as buffer, tri-state output), 74LS574 (8bit, tri-state).

The very interesting are 74LS375 (two 2 bits registers, similar to 74LS75 with different pinout), 74LS259 (eight 1bit latches - one input, 8 outputs) and 74LS670 (four 4bit registers with separated writing and reading).

Not recomended are: 74LS373 and 73LS374 due to their pinout.

Other interesing functions

The most advanced TTL chip is arithmetical unit (ALU) - 74LS181. It operates on two 4bit inputs and can perform 16 functions on them, giving 4bit output and set of extra flags.

Another interesting chip is 4bit adder - it can add two 4bit values. Probably the most common usage would be increasing a number by 1. (See Sam Coupé memory paging.)

Files for download

74ls610a.png 17.8kB (2005-05-08 22:59)
The LS74610 -74LS613 System Block Diagram. How the memory mapper works in example system.
74ls610b.png 35.4kB (2005-05-08 22:57)
LS74610 -74LS613 logic diagram.
d74ls.pdf 254.6kB (2005-05-08 22:45)
Symbols of many 74LS chips.
74ls469.pdf 120.9kB (2005-04-11 17:08)
74LS469 (NS). 8bit up/down counter with load and 3-state output.
74ls245.pdf 53.4kB (2004-11-08 14:43)
74LS245 datasheet from Motorola.
74hct670.pdf 77.6kB (2004-10-09 12:37)
74HCT670 datasheet. 4 parallel 4bit latches with independent writing and reading.
ucy74s416.pdf 314.8kB (2004-09-26 05:09)
[pl] UC74LS416 (intel 8216). Something similar to 74*243. Poor quality scan.
74ls251n.pdf 58.7kB (2000-03-21 15:21)
74LS251 datasheet from Fairchild.
74ls378.pdf 726.1kB (1999-12-07 15:28)
74LS377, 74LS378, 74LS379 - rare and expensive D-type flip-flops (4, 6, 8 bits). Similar to LS273, LS174, LS175, but common enable replaced common clear. Texas Instruments.
sdls132.pdf 155.2kB (1999-11-18 20:12)
74LS132 datasheet from Texas Instruments.
74hct574.pdf 56.2kB (1990-10-01 12:52)
74HCT574 and 74HC574 datasheets from Philips.

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© 2009-12-28 21:50 Jarek Adamski, http://8bit.yarek.pl