5158Re: [cosmacelf] Is there a Hex Keypad standard?
- Feb 1, 2007Ted,
Quest and Netronics followed the PopTronics article by
using a 74C922/923 keyboard encoder, along with a set
of latches (74C173 or similar). The 74C923 encodes a
keyboard press, has built in debounce, and presents
the hex result on 4 or 5 lines, along with a strobe
signal. The strobe is used to latch the previous
value to give 8 bits. These two parts are isolated
from the data bus via 4016 switches. The keyboard
encoder is getting hard to find, but Jameco still
The Elf II schematics can be found here:
--- ted_rossin <ted_rossin@...> wrote:
> I built an Elf from the Popular Electronics 1976http://www.geocities.com/ted_rossin/Electronics/RCA/RCA.html
> article back in
> 1978 and around 1979 I added a hex key pad in place
> of the binary
> interface (I used a solderless breadboard instead of
> the 8 switches
> for cost reasons).
> Click on the picture for a larger view.
> My keypad is getting flakey and I'm thinking about a
> redesign and
> thought that if there is a standard then I should
> adhere to that.
> The following is a description of how my current
> keypad behaves.
> My keypad drives the INPUT switch after every 2nd
> key press. This
> also drives /EF4 and /DMA_IN low and drives 8-bit
> data onto the data
> bus in response to the ELF doing a Port 4 read. I
> think the INPUT
> to /EF4 and /DMA_IN and port 4 read is standard ELF
> behavior when
> the machine is in the LOAD state.
> I also have my keypad drive /EF3 low when any key is
> and /EF4 low when the LOAD button is pressed. I
> have a toggle
> switch that selects between the LOAD button and the
> auto-load on
> every 2nd key press.
> This is my main user interface. Basically, I can
> look for EF3
> active and then read port 4 to see which key is
> pressed under
> program control or use the thing in auto-load mode
> to load programs
> starting at location zero. The later is how I jump
> to my monitor
> code that is held in ROM at location 0xf000.
> The keypad also has 8 LEDs connected to PORT 4 as I
> could not afford
> the HP hex displays back then.
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