Author Topic: How many inputs can a CMOS BCD output drive?  (Read 675 times)

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Offline cdreimer

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How many inputs can a CMOS BCD output drive?
« on: February 28, 2017, 02:19:57 am »
My 5V circuit uses a 4518 (BCD Up Counter) to 4511 (BCD to 7-Segment Driver). I want to add a 4028 (BCD to Decimal Decoder) to the BCD outputs of the 4518.

Does anyone know what the fanout is for a BCD output to BCD inputs? Every BCD circuit design I've looked at has one-to-one relationship between outputs and inputs.

This webpage claims that each CMOS output can drive up to 50 inputs.

https://electronicsclub.info/cmos.htm

One data sheet for the 4518 claims that the fanout can drive two 74L or one 74LS.

http://www.jameco.com/Jameco/Products/ProdDS/13565NSC.pdf

I got the 4518/4511 wired up on the breadboard. I haven't added the 4028 to the circuit yet. I'm trying to think this through before I blue smoke another IC.   :horse:
 

Offline MK14

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Re: How many inputs can a CMOS BCD output drive?
« Reply #1 on: February 28, 2017, 02:42:32 am »
You will be fine. It (the CMOS output) can handle many CMOS logic inputs.

The "74L or one 74LS.", are old logic gate types, which used quite a lot of input current (relatively), hence the low fan out for them.

EDIT:
The "low fanout" I was referring to, is when a relatively weak CMOS output, tries to drive TTL, such as 74 LS inputs.
TTL to TTL has better fan-out.
« Last Edit: February 28, 2017, 03:03:57 am by MK14 »
 
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Offline Brumby

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Re: How many inputs can a CMOS BCD output drive?
« Reply #2 on: February 28, 2017, 02:49:10 am »
The original 7400 series TTL had a fan out of 10 - for other TTL inputs.  The number for CMOS to CMOS is a lot higher.
 
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