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Lower Memory - Why Multiple Voltages?

ramsrcramsrc
edited October 2012 in Hardware
Having spent much of the weekend working on getting my old Spectrum working, my Dad (who is visiting) and I were chatting about the schematic.

Given that the upper RAM in a Spectrum is of type 3732 - which only has a supply voltage of +5V, why on earth did Sinclair use the 4116 in the lower portion of RAM?

Surely it would have been easier to have also used the 3732, supplied the whole machine with 12v to run the LM1889 and then the 7805 to derive the 5v.

Am I missing something? After all, the internal power supply and the 4116s seem to be the achilles heel of the Spectrum
Post edited by ramsrc on

Comments

  • mcleod_ideafixmcleod_ideafix
    edited October 2012
    The only reason I can think about is because it was the cheapest option back in the day. Remember that the Spectrum was born as a 16K-only machine, with the 32K daughter board coming a bit later, so I assume Sinclair already had supplies of 4116 chips, very common in some other designs, as the MSX, Dragon, and some arcade videogames. 4532 chips, on the other way, are no less than damaged 4164 chips, that TI and OKI packaged as "32Kbit chips" because one of the two 32Kbit banks was still usable.

    It was not uncommon to design part of a device, or the whole thing, to match the specifications of a component that the manufacturer had previously acquired as a bargain in a wholesale. Somewhere I read about the Vectrex videoconsole was designed around the Samsung 240RB40 B&W tube mainly because MB had access to a great supply of said tubes for a bargain.
  • 1024MAK1024MAK
    edited October 2012
    4116 chips were the most cost effective way to provide 16k of RAM when the Spectrum was designed (the 16k RAM pack for the ZX81 used them).

    The voltage inverter circuit is also cheap, and as +12V was needed for the video circuit anyway...

    Supplying the 5V regulator with +12V would have also made it run even hotter...

    The higher capacity 64k RAM chips were much more expensive. Indeed computer manufacturers used 32k chips, some of which were 64k chips that were not fully functional, so were sold as 32k chips by the semiconductor companies.

    Sinclair could have used only 32k or 64k RAM chips but then the whole of the RAM would have suffered memory contention (the ULA has to access the RAM to generate the display).

    Mark
    Sinclair FAQ Wiki
    Repair Guides. Spanish Hardware site.
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    former Meulie Spectrum Archive but no longer available :-(
    Spectranet: the TNFS directory thread

    ! Standby alert !
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    Looking forward to summer in Somerset later in the year :)
  • SpiranthoSpirantho
    edited October 2012
    They could have used 4816 chips like the BBC used, but I'd guess they were more expensive (Acorn not worrying because they weren't aiming at the lower price point). I imagine the 4116 chips were older because they were common in older more expensive machines, so they'd be cheaper.
  • ramsrcramsrc
    edited October 2012
    Thanks for the replies! I guess that makes sense - it was just a case of the best tool for the job at the cheapest price.

    Either way, the Spectrum is a masterpiece of Engineering. Who would have guessed 30 years ago that we'd still be using them today.
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