Interfacing TTL and CMOS logic

[terry michaels]

Message text written by jbv
>Hi there,

I'm back to this list with a slightly off-topic question.

I need to drive a logic CMOS IC from a TTL source (actually a port of a
8051).
The power supply of the TTL source is +5V, while the CMOS IC is +15V.
Of course, direct interfacing is impossible, especially because the
'positive' level of the TTL
output (typically 3.5V) is not sufficient for the CMOS. So I need to put
some buffer in between.

I found out that the 4049 (sextuple buffer) converts signals up to +15V
into +5V TTL regular
signals. So I imagine that there must be some equivalent buffer to
convert 0/5V TTL signals
into 0/+15V CMOS signals. And I don't have any CMOS databook at hand
(everywhere I ask, it
seems to be unavailable or sold out...)

Important : I have very little room left on my PCB, and that's why I'm
looking for a one-chip
solution (no extra resistors, etc.).

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

<

Hello:

One way to do this is to buffer your TTL source with an open collector TTL
buffer, such as 74LS641 or 642, then run the output of the buffer into the
CMOS, connected to a SIP resistor pack to +15 volts.  You probably can't
pull up the 8051 output to 15 volts.

Terry Michaels