I have two identical circuit boards using IC MM74HC123 one manufactured by Fairchild Semiconductor the other by Philips. The Fairchild chip when used in the circuit will announce any length of message the Philip circuit won't announce the longer messages. I don't have a circuit diagram for the board. I only difference on the datasheet is differences in the propagation delay, output transition time and power dissipation capacitance. Could anyone please tell me which of these would course the problem and why?

 

Hello,

What is the powersupply voltage of the boards ?
The HC series may work from 2 to 6 volts.
The HCT series may work from 4.5 to 5.5 volts.

Greetings,
Bertus

 

Hi,

The boards voltages 5 volts

 

I have two identical circuit boards using IC MM74HC123 one manufactured by Fairchild Semiconductor the other by Philips. The Fairchild chip when used in the circuit will announce any length of message the Philip circuit won't announce the longer messages.

Are you talking about the chip or the boards? How can you be sure the boards are identical?

John

 

Are you talking about the chip or the boards? How can you be sure the boards are identical?

John

The boards are identical, if I swopped the chips round the problem follows them not the boards

 

Then one chip is problematic and needs replacement. Take the good chip and check it on both boards to see if they are both good.

 

Then one chip is problematic and needs replacement. Take the good chip and check it on both boards to see if they are both good.

I have done this, and ordered replacement Fairchild chips, but I was just wondering why the problem was due to the Philips chip and if the higher maxium propergation delays and lower power dissipation capacitance would explain this

 

Maybe! What board is it? Can you post links for both the datasheets of the Fairchild and Philips chips to compare them?

 

Maybe! What board is it? Can you post links for both the datasheets of the Fairchild and Philips chips to compare them?

It's a memory interface board in part of a digital voice annoucement system.

Philips datasheet
http://docs-europe.electrocomponents.com/webdocs/002f/0900766b8002f0f1.pdf

Fairchild datasheet
http://www.farnell.com/datasheets/67047.pdf

 

One thing I noticed are Vref1 and Vref2 in the phillips schematic of the IC aren't exposed or described. If Vref1 and Vref2 of the schmitt triggers differed from chip to chip your Rext and Cext values may not be chosen correctly for such interchangeabaility.

 

If you compare the datasheets for the Phillips 74HC123 and the Fairchild 74HC123 you will find that the formula for the Phillips one-shot's pulsewidth is T = 0.45*R*C while the formula for the Fairchild 74HC123 is t=R*C. This difference is enough in many cases to prevent one manufacturer's device from replacing the device from another manufacturer. There is another semiconductor manufacturer's 74HC123 whose formula is t=0.7*R*C.

It is for this reason that the company I work for has a strict ban on the use of 74HC123 in any of our designs.

hgmjr

 

Can you scope the faulty board and see if the Philips part is producing a pulse?

I agree with hgmjr a "xx74xx123" is not the same as another "xx74xx123". Working with 1-shots can be frustrating if / when the circuit is designed to work with a certain pulse width.

-Sparky