Front panels

Dave Wade dave.g4ugm at
Sat Oct 31 17:52:05 CDT 2015

On my Baby Baby

I used three of these

that is the MCP23S17 as the inputs. A nice chip that’s 3.3v so compatible with modern FPGA and Raspberry PI and you get 16 lines per chip that can be set as in and out. 
You can cascade up to four chips on a 4-wire interface as these can be addressed by a 2-pin address input.

Very Nice


> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at] On Behalf Of Bob
> Rosenbloom
> Sent: 31 October 2015 22:35
> To: General Discussion: On-Topic and Off-Topic Posts
> <cctalk at>
> Subject: Re: Front panels
> On 10/31/2015 2:15 PM, Jon Elson wrote:
> > On 10/31/2015 03:11 PM, Charles Anthony wrote:
> >> What I don't know how to do is drive 500 or so LEDs.
> >>
> >> I am guessing a bunch of shift registers, but I've pretty much
> >> reached my design limits. I need some guidance on locating and
> >> understanding the technology to run that many LEDs.
> >>
> > it really isn't that complicated.  The simplest might be a byte shift
> > register, ie. a bunch of octal D-FFs like the 74HC374.  Given a
> > byte-wide group of GPIOs on the Beagle Bone, you could send out 63
> > 8-bit words with one additional GPIO to act as a clock for the FFs.
> > The LEDs could be driven directly from the FF outputs with a resistor.
> > With the current generation of high-efficiency LEDs 10 mA would be
> > plenty of current, and so the FF outputs would still be close enough
> > the specs to drive the next stage.  One downside of this scheme is if
> > the serial transmission was slow, you'd see a blink each time the Bone
> > sent a new light pattern.
> >
> > If you want to get more complicated, you could have one HC374 for the
> > shift register and one HC374 as the latch.
> > You'd shift all 63 bytes through the byte-shift register, pulsing the
> > byte clock 63 times, and then pulse the latch clock once to latch all
> > the 5xx bits of light info into the latch register, which would allow
> > the LEDs to be updated without any flash as the shift reg is being
> > shifted.
> >
> > Now, another way to do this is with multiplexing.  You could maybe
> > have 8 64-bit words that loaded to a small RAM, and the RAM is scanned
> > to load data to banks of 64 LEDs.  This reduces the number of drivers
> > to, say, 64 cathode drivers and 8 high-current anode drivers, but
> > complicates the rest of the thing a fair bit.  It will also cause the
> > whole panel of LEDs to flicker at the multiplexing rate, which could
> > be annoying when you flick your eyes across the panel.
> >
> > Jon
> >
> The 74HC595 8 bit shift register has a storage register also. You can cascade
> them then update the displays with a single pulse. I'm using them with
> ULN2803 8 bit drivers to drive the incandescent bulbs on my 360/30 panel.
> Bob
> --
> Vintage computers and electronics

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