Table of Contents

Components Compatibility and Substitution


Components substitution seems to be one of the repeating topics on N8VEM list. Hopefully this document will clarify some commonly asked questions.

Logic ICs

Most of N8VEM and related projects were designed with 74LS (low power Schottky transistor transistor logic) in mind. While this logic series is widely available, other alternatives exist, and can be used to substitute this logic series.

The two classes of logic, TTL and CMOS, are significantly different in their fundamental operation: TTL is current driven, while CMOS is voltage driven.

TTL (Transistor Transistor Logic)


The 7400 series was the original TTL logic series. It was defined by a 16ma drive capability, and the necessity to sink 1.6ma at each input. Hence, it had a fanout of 10, meaning that one gate could drive 10 inputs. A typical gate delay for this series is about 10ns.

7400 variants - 74H (high speed), 74L (low power)

These families of TTL logic were never widely used. The 74H series used even more power than the 7400 series, and the 74L series, although requiring less power, was quite a bit slower.

74S - Shottky TTL

Shottky logic replaced the 74H series. It uses a great deal of power, all chips run hot, but it is very fast. Typical gate delays are around 3ns, output drive around 20ma, and input sink current requirement of around 2ma. Hence it has a fanout of 10. Until the introduction of 74F logic, the 74S series was the choice where sheer speed was the consideration.

74LS - Low power Schottky TTL

Power requirements were greatly reduced with the introduction of 74LS logic. Although drive capability was reduced to about 8ma, the input current sink requirement was greatly reduced to around 0.4ma. Hence, LS-TTL has a fanout of about 20. Speed is comparable to the 7400 series, a typical gate delay is about 10ns. It essentially replaced the 7400 series.

The 74LS series has for many years been one of the most widely used logic series.

74ALS - Advanced low power Schottky TTL

74ALS is one of the best replacements for 74LS. It is about twice as fast as 74LS, typical gate delays being about 5ns. Drive capability is the same, around 8ma, but the winning characteristic is the input current sink requirement of no more than 0.1ma. Normally 74ALS can be used as a drop-in replacement.

74F - Fast TTL (or F = Fairchild)

The 74F series, originally introduced by Fairchild Semiconductor, is about as fast as 74S logic, but consumes just a little more power than 74LS. Typical gate delays are in the 3-4ns range, output drive is about 20ma, and the input sink current requirement is about 0.6ma. For high speed, it has largely replaced the 74S series.

** 74AS - Advanced Schottky TTL **

This series was introduced as an alternative to 74F logic. Just as 74ALS was a lower power version of 74LS logic, 74AS is a lower power version of 74S logic.

Substituting for 74LS

74ALS lowers power requirements, and is less expensive from many suppliers. 74F is the choice if speed is really needed. Any of these families may be substituted if you run out of a particular 74LS part.

One caution: 74LS is often the only family with hysteresis on bus receiver inputs. This makes 74LS14, 74LS132, 74LS240, 74LS244, 74LS245, 74LS682, and 74LS688 the preferred chips for bus receivers. As drivers, any of the above may be substituted. One exception to this rule is 74ALS14 – it has hysteresis on its inputs.

However, the N8VEM ECB bus seems not to suffer from noise problems, and I have not heard of anyone having problems with substitutions for bus receivers.

CMOS (Complementary Metal-Oxide-Semiconductor) Logic

CMOS logic became popular in late 80's as its performance became comparable, or even better than performance of TTL logic. Multiple manufacturers produce pin compatible CMOS versions for 74-series ICs. It should be noted that standard CMOS logic levels are not fully compatible with TTL specifications: The minimal valid “high” voltage level for plain CMOS logic is 3.5 volts, while TTL accepts voltages as low as 2.7 volts. It is not recommended to mix standard CMOS logic (74HC, 74AHC, 74AC) with TTL or NMOS devices due to voltage level differences. There are special CMOS logic series, like 74HCT, 74AHCT and 74ACT that have TTL compatible inputs and generally can be used as a replacement for TTL logic.

There are some differences between TTL and CMOS devices, that should be considered when replacing TTL devices:

74HCT - High speed TTL-compatible CMOS

The speed of 74HCT is comparable to the speed of 74LS, but it is not always the same. Some 74HCT devices are faster than 74LS counterparts, but some of them are slower. But in general 74HCT could be used as a replacement for 74LS counterparts.

74AHCT - Advanced high speed TTL-compatible CMOS

74AHCT are about 2-3 times faster than 74HCT, and have similar power requirements. They are also a good replacement for 74LS and 74ALS parts.

74ACT - Advanced TTL-compatible CMOS

74ACT devices are mostly intended for driving higher loads (e.g. bus signals). They have significantly higher output current of 24 mA, compared to 8 mA for 74HCT and 74AHCT. Their performance is close to 74AHCT, but they are more power-hungry. Another difference of 74ACT is fast edges (switch times). While in some cases it could be good, generally when replacing TTL, it results in more noise.


Programmable logic device ICs

Simple programmable logic devices

PAL, PLA, PLD, SPLD, etc. Alas, many of them are one-time programmable. Can the erasable-and-reprogrammable GAL and EEPLD devices be programmed to be backwards compatible?


“Atmel and Altera CPLD Compatibility Notes” (cpld-compatibility)

ASICs (Z80, Intel 8xxx, UART)

Most CMOS ASICs are fully compatible with NMOS counterparts and have much better characteristics - significantly lower power consumption and higher speed.

Known working substitutions

MAX232 RS-232 Driver

Pin-compatible ICs are manufactured by multiple companies. Some of them use 0.1 uF capacitors, or can work with either 1 uF or 0.1 uF capacitors. 0.1 uF ceramic capacitors can be more convenient to use than electrolytic 1 uF capacitors. They don't have aging problem, they suffer less from overheating (e.g. during soldering), they don't have polarity, they are smaller.

Here are some MAX232 compatible devices:

Passive Components

Resistor Arrays

Resistor arrays can be replaced by discrete resistors soldered together. Some boards don't use all resistors in the array, so you can save some resistors.

Power Supplies

N8VEM boards use 5 volt only power supply. But other voltages, e.g. 12 volt can be required to power other equipment, like hard drives. Make sure to use regulated power supply, also double check the power supply polarity.

For powering a single N8VEM or Zeta SBC it is usually enough to use a small wall power adapter, rated for 500 mA to 1 A. For more boards or additional peripherals a PC ATX power supply can be used. Connect on/off switch between green wire (power on signal) and black wire (ground) for turning on the power supply.

Jameco, Mouser and other distributors also sell smaller (than ATX) switching power supplies, that provide 5 volts and 12 volts (and some models also -12 volts) with 5 A or higher for 5 volt line. It is recommended to use enclosed-style power supplies - they are more safe to work with and generate less EMI.

Personal Experience




74 Series Logic IC - Includes some generic specifications for 74LS and 74HCT series.

FAST and LS TTL Data - Motorola (Local Copy)

An Introduction to and Comparison of 74HCT TTL Compatible CMOS Logic - Fairchild Semiconductor

HCMOS Design Considerations - Texas Instruments