I'm aware that 40-pins was a high-water mark for dual-inline package chips for a significant time; many CPUs of the early 1980s (8086, Z80, 6800, 6502, etc) used 40-pin packages, but no larger.

Likewise, general purpose I/O chips were frequently in 40-pin packages. Examples I can think of off-hand include Intel's 8255, the MOS 6522, and National Instruments' INS8154. The drive to offer as many pins of I/O as possible on your chip seems perfectly reasonable, given that your competitor would also be doing so.

A number of variants of the 6502 were manufactured in smaller (28-pin) packages with reduced address bus width and/or other changes. This suited the use of 650x chips in embedded devices, where a smaller chip could be more convenient. But I'm not aware of any general-purpose I/O chips that were made available in a reduced package; to my knowledge you'd still have to use a big 40-pin I/O chip alongside your little 28-pin processor.

Were there any general purpose I/O chips manufactured in a smaller DIP package than 40-pin?

  • 3
    For context, this question was prompted by an old (1970s) prototyping system that doesn't support DIP chips larger than 36-pin.
    – Kaz
    Feb 13 '20 at 13:00
  • 3
    What is included in I/O controller chips? Does a chip like the Z80 CTC fall into this category?
    – UncleBod
    Feb 13 '20 at 13:05
  • 1
    @UncleBod I'm looking for general-purpose programmable I/O, such that I can switch the logic level of specific (output) pins in software at will, and read the value of specific input pins too. Something with the flexibility to scan a keyboard matrix, drive an LCD display, or control other logic chips, as the designer wished. From a glance at the Z80 CTC, it could be used to do some of the above, but it's not really suited to the task; it's designed to be used as a set of counters/timers.
    – Kaz
    Feb 13 '20 at 13:14

I nominate the AY-3-8912, which is a sound generator that also has eight programmable input/output pins — set them as input or output, set or get their level — in a 28-pin package. It sits between the 8910 (16 IO lines, 40-pin package) and the 8913 (no IO, just the audio, 24-pin package).

  • The AY-3-8912 sounds* like just the kind of chip I was thinking about, thanks. *no pun intended!
    – Kaz
    Feb 13 '20 at 16:07

Of course. After all, the number of pins is related to the task at hand. No manufacturer would choose a package with more pins than necessary.

Examples of main families are:

(Excluding support chips, like priority encoder, status decoder or bus drivers)

  • Motorola 6800 family

    • MC6840 - PTM - Programmable Timer (28 pin)
    • MC6850 - ACIA - Asynchronous Communication Interface Adapter (24 pin)
    • MC6852 - SSDA - Synchronous Serial Data Adapter (24 pin)
    • MC6854 - ADLC - Advanced Data Link Controller (28 pin)
    • MC6859 - DSD - Data Security Device (DES en-/decoder) (24 pin)
    • MC6860 - 0-600 bit/s Digital Modem (24 pin)
    • MC6862 - 2400 bit/s Modulator (24 pin)
  • Intel MCS-80 family

    • 8212 - 8 bit I/O port (24 pin)
    • 8251 - UART - Programmable Communication Interface (28 Pin)
    • 8253/54 - PIT - Programmable Interval Timer (24 Pin)
    • 8259 - PIC - Programmable Interrupt Controller (28 Pin)
  • MOS 6500 family

    • 6551 - ACIA - Asynchronous Communications Interface Adapter (28 pin)

Only chips that need to handle/produce many signals, like DMA, or FDC, had multiple functions, like (R)RIOT, or those that were all about offering as many as possible (parallel) ports, were given 40-pin chips.

In addition it's important to remember that simple ports can be (and usually have been) done using basic TTL chips. After all, being TTL compatible is a main feature of early microprocessors. They come with the least amount of pins and lowest cost at the same time. For example:

  • A 74173 gives a 4 bit buffered input in DIP16
  • A 74241 does two 4 bit input ports in DIP20
  • A 74374 offers 8 bit input with buffering/edge triggered in DIP20
  • A 74273 makes an 8 bit output port in DIP20

All of them in 300 mil package, saving quite a lot on space. For a system designer it doesn't make much sense to use an more expensive higher integrated chip to get a functionality that can be achieved with standard logic as easily.

Bottom Line: It's all about function and complexity.

  • It's those multi-function, general purpose I/O chips that I'm interested in, the ones where designers would usually want to offer as many I/O pins as possible. (I may not have made that prominent enough in my question, and I'll be editing it accordingly. If there's a better name for the type of chip I'm trying to describe, please let me know.)
    – Kaz
    Feb 13 '20 at 14:58
  • @Kaz Come on, then the question doesn't make sense. Why on earth should anyone want multiple functions, but without enough pins to use them?
    – Raffzahn
    Feb 13 '20 at 15:01
  • for the same reason someone would want a 650x microprocessor, but not want it in a 40-pin package? CPUs were available in packages with reduced functionality, did the likes of a MC6820 or 6522 exist in a cut-down form? From the current answers, GI thought there was a market for the 24-pin AY-3-8912 in-between the 8910 and 8914, with a bit of general-purpose I/O, but not as much as they could fit on a 40-pin chip.
    – Kaz
    Feb 13 '20 at 15:07
  • The 8912 isn't an inbetween, as the 8914 is basically the same 40 pin as the 8910. The 12 is simply a package reduced one for just the sound. Since there is no 20 Pin 600 mil standard size to be used, 24 would be minimum, making 4 pins NC (or a 4 bit port), while using the only slightly larger 28 pin allows a full 8 bit port. Logical choice. ... also, did you notice the 8212?
    – Raffzahn
    Feb 13 '20 at 15:19
  • Apologies, the other answerer described the 8914 as a 24-pin package; from a Wikipedia search I think they may have meant the 8913. The Intel 8212 is the best match (imho) from the devices you've listed. A read of it has set me wondering, given how similar to a basic 8-bit latch the 8212 is, how big a market would there have been between 40-pin general-purpose I/O chips and doing it all yourself with a handfulof TTL chips.
    – Kaz
    Feb 13 '20 at 15:34

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