Mixing 4000 and 7400 Series

[Michael]

Hi,

I have a circuit that is made up of 4000 series CMOS ICs but it needs a
magnitude comparator to work and I only have the 74LS85. Is there anyway I
can use this so I don't have to order the 4063 CMOS comparator?

Thanks in advance,

Michael

 

[[email protected]]

Hi,

I have a circuit that is made up of 4000 series CMOS ICs but it needs a
magnitude comparator to work and I only have the 74LS85. Is there anyway I
can use this so I don't have to order the 4063 CMOS comparator?

Thanks in advance,

Michael

Your 4000 series slows down at 5V and your LS blows up over 5V. Is
that acceptable?

Mixing logic families is mostly OK but things like reset/preset are
often logic low for TTL and high for CMOS along with limited drive
from CMOS. I'm sure most everyone has done it at some time. I know I
have.

GG

 

[Michael]

Hi,

I have a circuit that is made up of 4000 series CMOS ICs but it needs a
magnitude comparator to work and I only have the 74LS85. Is there anyway I
can use this so I don't have to order the 4063 CMOS comparator?

Thanks in advance,

Michael

Your 4000 series slows down at 5V and your LS blows up over 5V. Is
that acceptable?

Mixing logic families is mostly OK but things like reset/preset are
often logic low for TTL and high for CMOS along with limited drive
from CMOS. I'm sure most everyone has done it at some time. I know I
have.

GG

Thanks,

The circuit is spread across multiple PCBs with this CMOS/TTL problem on
it's own board. So in theory I should (and can) increase the supply voltage
on the CMOS bits of the circuit and keep the TTL parts still at 5V? That
should reduce the operating speed difference somewhat correct?

There's no reset type pins it's just purely data that will be sent between
the two that makes thing's slightly simpler.

Cheers,

Michael

 

[whit3rd]

I have a circuit that is made up of 4000 series CMOS ICs but it needs a
magnitude comparator to work and I only have the 74LS85. Is there anyway I
can use this so I don't have to order the 4063 CMOS comparator?

A 4000 series CMOS gate will drive ONE 74LS series standard
input adequately, provided (1) the CMOS has 5V power supply
and (2) the two chips are at the same temperature.
CMOS sinks inadequate current when it's hot for a 74LS
input when that LSTTL chip is cold.

And, an output from a 74LS gate will drive a CMOS gate adequately
if you add a pullup resistor (for highest performance, 4k ohms to +5V).