hexkeyboard to BCD/Binary

Is there any IC or a simple circuit that convers hex keyboard inputs into binary or BCD ?

Kindly help me out...

Thanks in advance.

if you only need 0 to 7 you could use an 74HC138 (3 inputs, 7 outputs)

Added after 39 seconds:


i mean 8 outputs

I wanted to make a keyboard to directly write into the ram for my 8085 board...

So I must have 16 outputs....

I have reasons not to use the existing chips from Intel...

Thanks, I will think if 2 such ICs help....

You can also use an 74HC4067, just tie the control lines to the hex keyboard and the input to ground, use pull-up resistors for ouputs, and the selected output will be 0V . Or the other way around (Input to GND and pull-down resistors on outputs).
That works for the HEX to BCD, but i don't understand how do you want to write directly to Ram, don't you need an BCD to HEX converter ?

Adi

A 74C922 will do the job, but it might be expensive:
**broken link removed**

/Rambo

An idea that occured to me was...to use two octal to BCD and a full adder...

If the key 8 is pressed the 2nd encoder would generate 0001 which shall be added to 7(7 shall be hardwired to the full adder) to give 1000 and if F is pressed,

1000 + 0111 =1111 !!!

What do you guys think ?

I wouild not have to take this trouble if I knew of any HEX to BCD encoder....

Here is a discrete circuit that will do what you ask for:
**broken link removed**

XNOX_Rambo said:

Here is a discrete circuit that will do what you ask for:
h**p://www.mitedu.freeserve.co.uk/Circuits/Misc/dec_bcd.htm

Click to expand...

Thanks a lot.... The site does set me on the track....

I really would like to know where you got it from ? I search google and I never found it... I know google is not the only search Engine...But I give up all hopes when Google does not deliver results....Please give me a few search tips...


Also is there a way to deal with the zero/F problem ? The TTL devices assume open input as high. So when No key is pressed, all 4 lines seem in a high impedence state is(1111==F)....So looks like I have to store data in a register and then send it to the RAM when I am sure that the signal is due to a key pressed and not high impedence 1111....

Any easier way around it ?

The secret to searching the net is using a couple of words and trying to think
of unique ones. E.g. in this case - hex bcd keypad. Note that "keypad" is a better
word than "keyboard" in this case. Also try to exclude words that you know will
trigger false hits.
I use https://www.eniro.se to search and it performs well for me. It's a Swedish site
but you only need to know that "sök" means "search" in swedish. :wink:
Use quotes to search for sentences: "hex to bcd keypad"
And use the minus sign to exclude words: -software

As for the 1111 problem - one should never leave logic inputs floating to avoid
false values. You can either use a pull-up resistor to Vcc to set the input high,
or pull-down resistor to GND to set it low.
In this case four pull-down resistors to GND will give you a 0000 value. You can
use e.g. 10 kOhm for the resistors - it is not critical.

/Rambo