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These days it's mostly converged on USB, but in the old days computers tended to have lots of different kinds of ports. In particular, I'm interested in:

  • 9-pin serial
  • 25-pin serial
  • parallel printer
  • external disk drive
  • Atari joystick
  • IBM keyboard
  • Ethernet

How much did each of these kind of ports cost to add to a 1980's home computer, in the old days? If exact figures aren't available, I'd be interested in an overall order of magnitude figure plus some idea of the differences, e.g. how much cheaper is 9-pin serial compared to 25-pin?

I'm specifically interested in Z80-based computers (e.g. ZX Spectrum).

Okay, for more specificity: yeah, ZX Spectrum is a representative particular machine, and I'm interested in total extra cost, the connector plus any chips needed to drive the port that weren't already necessary in the machine. I'm only interested in variable cost, not fixed, so the cost of developing any driver software can be ignored (though if it would be large, the cost of ROM space can't).

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    The only difference between 9-pin serial and 25-pin serial is in the connector (all signals are the same, many pins remain unconnected), so the difference in cost is neglegible. Note that 25-pin serial was standard for a long time, and the 9-pin serial was introduced at some point in the PC era IIRC, which is why you had often both options (for compatibility) for quite some time. As for prices in general, I can't help you. – dirkt Apr 25 '17 at 5:22
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    The physical connectors were more expensive back then, but big expense was often with the extra chips needed. An RS-232 serial port would require a UART and a pair of MC1488/1489 driver chips, along with a +/- 12V power supply. That's why the C64 didn't have one, it bit-banged a TTL level serial connection through its user port edge connector. Did the same thing for it's serial connection to the disk drive and printers. – user722 Apr 25 '17 at 5:52
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    One way of getting some idea of the total cost of the ports with their associated electronics would be to find the cost of PC expansion boards with the ports, in isolation (serial, parallel, IBM PC joystick and Ethernet). – Stephen Kitt Apr 25 '17 at 9:46
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    In some (especially newer) cases the cost is exacerbated by patent/license fees, often connected with certification fees (to include certain interface in your device you need to get it certified with the standard's owner, and pay a fee per each device on top of hefty certification fee.) This isn't as common with "legacy" interfaces. – SF. Apr 25 '17 at 10:25
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    An external disk drive could have a marginal cost for the port of everything from zero (connected to parallell port, say) to hundreds of dollars or more for the simple case (external SCSI, for example). And that's just one example. Also, what do you consider to be "the old days"? Ten years hence? Thirty years? Fifty years? – user Apr 25 '17 at 14:16
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In 1985 I paid $50 for a parallel printer adapter for my Atari 800xl to connect it to a standard Canon printer. The most popular one at the time called an ape face was around $60. The Atari 850 interface which was both serial and parallel was coming down in price but was still over $100. The 1050 disk drive was around $180 while the non Atari drives that were true double density or quad density started at $350 up to $700. The hard drives at the time all came with their own interface boards that only worked with the specific drive that came with it. I remember 5 mb drives being around $1000 and Corvus which had an interface board that also networked Atari's together and some other functions I don't recall sold a 20 mb drive which was close to $3000. I remember many home brew type joystick interfaces that could use the basic Atari stick with many systems but I don't remember prices.

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