Shop OBEX P1 Docs P2 Docs Learn Events
Odd way to power 74LS chips? — Parallax Forums

Odd way to power 74LS chips?

Monitor LizardMonitor Lizard Posts: 5
edited 2013-11-14 02:07 in General Discussion
I was looking through some old schematics of late 70's era computers and noticed two 74LS244s being powered like so:

schema.png


What is the purpose of the inductor and resistor on the power rails? The chips add tri-state to a 6502 address bus.
schema.png
252 x 258 - 13K

Comments

  • tonyp12tonyp12 Posts: 1,951
    edited 2013-11-13 07:01
    inductor: to combat noise & spikes on power supply line

    resistor: fight against external ground bounce
  • Mark_TMark_T Posts: 1,981
    edited 2013-11-13 07:46
    tonyp12 wrote: »
    inductor: to combat noise & spikes on power supply line

    resistor: fight against external ground bounce

    Except that the inductor acts as anti-decoupling (perhaps a cap was missing from the schematic).

    The resistor might be used for allowing chip current consumption to be measured.

    I've a great little book called "Digital Hardware Design" by Catt/Walton/Davidson from the 70's
    endevouring to explain transmission line theory physics to electronics engineers - in it they convincing
    explain that much early TTL circuits only worked because of TTL's self-decoupling behaviour(*). You
    have to remember that a lot of early digital hardware was designed by RF engineers who only ever
    experienced the harmonic oscillator and resonant systems, not the heaviside step.

    (*) current switching transients are absorbed by nearby static loads in bias circuitry, reducing the
    severity of voltage transients on the supply. CMOS has no such robustness.
  • Monitor LizardMonitor Lizard Posts: 5
    edited 2013-11-13 09:54
    There is no decoupling cap on these two chips, which are the only two that use this power connection.
  • Beau SchwabeBeau Schwabe Posts: 6,566
    edited 2013-11-13 10:38
    "inductor: to combat noise & spikes on power supply line ... ...resistor: fight against external ground bounce"

    ... I think it's the other way around in this case TTL is a great source of noise of itself. If the supply line is shared with any analog circuitry or other noisy TTL's then the inductor will help to Decouple that noise. The resistor might take a little bit of the edge off of the noise generated from the TTL as well by lowering the "Q" factor of any harmonic oscillations that could occur. Remember, even though there aren't any physical capacitors there are parasitic capacitors that are formed within the IC mostly snubbed by a back to back parasitic diode effect within the TTL but still present.

    http://williamson-labs.com/480_byp.htm
  • Mark_TMark_T Posts: 1,981
    edited 2013-11-14 02:07
    Indeed, I'd assumed only digital circuitry as this was in the context of computers but indeed there
    might be analog peripherals nearby. Nice animation of the development of decoupling through
    the ages on that site, BTW :)
Sign In or Register to comment.