Re: New Elf called Mar1e - dumb question, before I start modding #Homebrew #Electronics


Thanks very much for this Lee. I have some blue LEDs in black holders which are quite bright at 0.5mA. I have a second 24 pin socket wired for a 6116 but not in use yet. I might make a plug-in board with eight LEDs to fit that. All the signals and ground are there.

Best regards,


On Tue, 9 Apr 2019 at 22:05, Lee Hart <leeahart@...> wrote:
John wrote:
> I really want to display the current address when I'm loading a program...
> If I want to latch the lowest 8 bits of the address bus and show them on
> LEDs, can I use a 74HCT373 octal latch with its strobe input connected
> directly to TPB?  ... I expect it's not that simple...

In the case of the 1802, it really *is* simple. :-)

To display the low 8 bytes of the address, all you need are 8
high-efficiency LEDs and a resistor network with 8 (or more) resistors.
Individual resistors can also be used. The 1802 can drive them directly
as long as the LED current isn't more than about 1 ma. I suggest
resistor values in the range of 2.2k to 4.7k.

Connect the common pin of the resistor network to GND.
Connect each resistor pin to each LED cathode.
Connect each LED anode to the 1802 MA0-MA7 pins.
The LEDs will light for any MA0-7 bit that is "high".

In LOAD mode, the address bus continuously outputs the low byte of the
address. TPA pulses and the high address byte are not present, except
during the actual DMA-IN cycle. So, you don't need a latch. :-)

If you also want to see the high byte, you need a 74HC373 latch as you
desscribed (connected to TPA), and 8 more LEDs and resistors connected
to the outputs of this latch as described above.

Lots of modern LEDs are efficient enough to be bright at 1ma. If your
DMM has a "diode test" range, this usually applies about 1ma as its test
current. Connect your LED to it to read the LED's forward voltage drop
at 1ma.

The red LEDs I supply with the 1802 Membership Card are highly visible
at 1ma, and they are just garden-variety parts. The "best" LEDs are
likely to be green or blue, which can be highly visible even at 0.25ma.

Fools ignore complexity. Pragmatists suffer it. The wise avoid it.
Geniuses remove it. -- Alan Perlis, "Epigrams on Programming"
Lee Hart, 814 8th Ave N, Sartell MN 56377,

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