Hello Robert.

After i look second time to your schematic i understood that you idea
have the
same misadvantage as John idea - the Sw_3 being pressed lock the other
keys.

It seems to be no way easily than John offer without disadvantages of
John's idea.
(impossibility to detect multi-key combinations)

WBR Dmitry.

> The schematic is correct, but I see now that I used a confusing convention
> for diode polarity. Thanks for pointing that out.
>
> I meant the "+" sign to indicate the direction current would flow FROM the
> diode when forward biased (instead of the more usual direction of current
> flow INTO the diode).
>
> Your way is more logical, though, so here's how the original circuit would
> look with the "+" representing the ANODE of the diode:
>
>                        Vcc
>   SW3                   |
>      __I__              R1
>                     +   |                           |
>  +---o   o---o----D3----o--------o----------o-------| B0
>  |           |                   |          |       |
>  R3          |             SW1 | o    SW2 | o       |
>  |           |                -|         -|         |
> Gnd          |                 | o        | o       |
>              |                   |          |       |
>              |         Vcc       |          |+      |
>              |          |        D1         D2      |
>              |          R2       |+         |       |
>              |      +   |        |          |       |
>              +----D4----o--------o----------o-------| B1
>                                                     |
>
> Again, the "+" indicates the direction current will flow INTO the diode
> when it is forward biased.
>
> With programmable pullups on B0 & B1, R1 & R2 can be eliminated. If you
> want to gamble that the code will never set either B0 & B1 as high outputs,
> you could even eliminate R3, though I wouldn't.