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.
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:
- Power consumption of CMOS devices generally much (5-10 times) lower than power consumption of TTL. This is usually a good thing.
- Speed (propagation delay) of CMOS devices depends on the capacitance on their outputs. This might be a problem when driving a high-capacitance signal (e.g. long PCB trace, bus signal, many inputs connected to the CMOS output).
- CMOS devices have symmetric sink / source output current. This is usually a good thing. For example it allows driving LEDs connected either between CMOS output and ground (lights up when output logic level is '1') or between CMOS output and +5V (lights up when output logic level is '0'). In comparison TTL sink current is much higher than source current. Standard 74LS components can sink up to 8 mA but source only 0.4 mA.
- CMOS devices are more sensitive to floating (unconnected) inputs. While TTL logic usually will assume unconnected input as logic “1”, in CMOS it might cause the gate to oscillate between “0” and “1”, resulting in increasing power consumption and noise. Make sure to connect unused inputs to either ground or Vcc. (It is a good practice to connect inputs of unused TTL gates inputs to the ground too).
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.
- 74LS Series bus drivers and transceivers, such as 74LS240, 74LS241, 74LS244, 74LS245 have hysteresis inputs (basically Schmidt triggers on inputs), newer TTL series ICs, such as 74F and 74ALS, and CMOS 74HCT, 74AHCT, 74ACT don't have it. This feature should reduce noise (ringing) during signal switching. It is not clear though that this feature is important for relatively slow ECB bus. Most ISA systems (with similar timing and frequencies) use 74F buffers and transceivers without any problems.
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
- CMOS Z80 CPU - Z84C00xxPEC or Z84C00xxPEG instead of NMOS Z80 CPU (Z8400). CMOS Z80 CPUs also available with speed up to 20 MHz.
- Intel 8255 PPI. Can be replaced by CMOS analogs: Intersil / Harris - CP82C55A (note CP82C55A is 8 MHz part, while CP82C55A-5 is 5 MHz part), Toshiba TMP82C55A, NEC D71055C. Latter two parts are available with speeds up to 10 MHz.
- 16550 UART. National Semiconductor PC16550D, Texas Instruments TL16C550, Exar ST16C550, California Micro Devices CM16C550. All 16550 UARTs are actually CMOS parts, even ones that don't have 'C' in the component name. Most N8VEM projects don't use UART's FIFO, so 16450, 16C450 and 8250 can be used also, but they are not recommended.
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:
- Analog Devices ADM202
- Exar SP202ECP (0.1 uF capacitors)
- Intersil HIN202 (0.1 uF capacitors)
- Intersil HIN232 (either 0.1 uF or 1 uF capacitors)
- Maxim MAX202CPE (0.1 uF capacitors)
- Maxim MAX232ACP (either 0.1 uF or 1 uF capacitors)
- Texas Instruments TRS202ECN (0.1 uF capacitors)
- STMicroelectronics ST232CN/BN (either 0.1 uF or 1 uF capacitors)
- Maxim MAX3232CPE (3×0.33 uF + 1×0.047 uF capacitors for 5.0V power or 4×0.1 uF for 3.3V)
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.
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.
- My N8VEM (SBC V1, Disk I/O, Bus Monitor), Zeta SBC, and XT processor board are mostly built using 74AHCT and 74HCT logic ICs. Notable exceptions are 74LS06 and 74LS92 that don't have CMOS equivalent.
- I had problems with using CMOS parts, 74AHCT373 particularly in my SBC-188 board. It resulted in flaky DMA (floppy) operation, replacing it with 74ALS373 resolved the issue. All other logic ICs are still 74HCT / 74ACHT.
- Problems with Dual IDE board then using CMOS (bus drivers?) ICs. Replacing with TTL resolved the issue. Other (non-bus) driver ICs are still CMOS.
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