Interfacing TTL and CMOS logic

[Fraser, Colin J]

> -----Original Message-----
> From: terry michaels [mailto:104065.2340 at compuserve.com]
> Sent: 12 March 1999 02:41
> To: jbv; synth
> Subject: Interfacing TTL and CMOS logic
> 
> I also read somewhere that pull-up resistors connected between the TTL
> outputs and the VCC
> would be a workable solution... But to which VCC : +5V for TTL ics, or
> +15V for CMOS  ics ?
> 
> Any suggestion ?
> 
> jbv
> 
> You probably can't
> pull up the 8051 output to 15 volts.

There is a way to use bi-directional ports on microcontrollers as open
collector outputs.
You write all zeros to the output register then use the data direction
register to control the output.
If a pin is configured as an input, it will float at high impedance - this
can then be pulled up to 15v with a resistor.
If it's configured as an output, it will pull down to zero volts.

Colin f