74HC123 and 74LS123

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A+B are two halves of the same chip. You only need one '123.

Look at the chip datasheet: Google for "74HC123.pdf" or "74LS123.pdf"

They are pin-compatible but the pulse width calculation is different: HC: PW = Rext * Cext LS: PW = 0.37 * Rext * Cext

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"74LS123.pdf"

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The HC123 can be used instead of the LS123 but you will need to adjust the value of Rext and/or Cext to get the same delay.

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The timing cap pins (6&14) need to be grounded on the HC: the LS will work without this connection. Wade

Wow, I didnt realize that it was just one chip. That changes things quite a bit. I suppose I should have seen that due to the fact that the "two" chips didnt re use the same pins. I just thought that they both did two different things and needed two chips. I suppose then the designation IC3 A + B on two chips in the diagram means "Use the same chip".

I did look at both the data sheets but Im afraid the information is beyond me. Do you believe that the one could be used with the other?

Thanks.

Wade,

Im not sure exactly what you mean. Are you saying that I should put it together as described in the diagram but also have a jumper wire go from pin

Please cut and paste into notepad.

Ground | | |---- 14 CEXT Pin | | == C2 47u + R4 R47k |

-/\/\/\-------15 Pin

Ground | | |---- 6 Pin | | == C3 1000u + R5 47k |

-/\/\/\-------7 Pin

Thanks.

Thanks Andrew, I see that note now.

Yes, that's what he's saying: connect pin 6 and 14 to ground. It says so, in a note, on page 1 of the HC datasheet:

Well last night I re did things using one 74HC123 chip and grounded 14 and 6 and it appeared to work.

Im not sure what you mean by 15 and 0. on Chip B. Since on Chip A 15 is already used up So it cant be used again on B and I'm unfamiler with pin 0 unless your talking SCSI language where 0 is the first device. But I dont think there is a pin 0, unless you mean pin 1? Which is again being used on Chip A.

In either case I think it is working for me now. Thanks.

Did you connect anything to pin 15 and pin 0 of the "B" chip. I have not looked at the datasheets but typically chips need power for their internal workings.

snip

Don't thank him too much :-)

On most TTL-ish 16 pin parts (including the '123), power is pin 16, not 15, and ground is 8. On a 14 pin part, power is 14 and ground 7 (still on the corners.)

--- Not always.

For Philips' HC it's Tw = K RC, where K ~ 0.45 for Vcc = 5.0V and ~

For TI LS, it varies from about 0.33 with Cext = 1µF to about 0.5 with Cext = 100nF

-- John Fields

--- The 74HC123 is a _dual_ retriggerable monostable multivibrator. That means there are two one-shots (monostables) in the same package, with common power supply terminals. Pin 16 is used to supply +5V to both one shots, internally, and pin 8 is used to connect both one-shots to ground (0V), internally.

Pin 15 is where the junction of the timing capacitor and timing resistor for one of the one-shots is connected, and pin 7 is where the junction of the timing capacitor and timing resistor for the other one-shot is connected.

Pins 6 and 14 are where the other ends of the timing capacitors are supposed to be connected, and even though they're connected to ground inside the chip it's a good idea to connect them to ground externally as well. For some manufacturers, like National, it's evidently a requirement. For others, it's suggested in order to lower the noise margin.

In your application, even though it's not shown on the schematic, it would be a _very_ good idea to put diodes across the timing resistors. The cathode should be connected to the power supply side of the resistor, and you should use Schottky (or germanium) diodes rated for a breakdown voltage greater than the supply voltage.

-- John Fields

and 6

on

Thanks jhomppi,

I will squirrel the information away. I will re read the datasheet to see how they describe pin 15. I find the data sheets (for good reason) overwhelming and filled with a lot of shortforms that get confusing. Even when they explain it I get lost but I trudge on. Sometimes little details like you explain help me to suddenly get " the gestalt Ah"

Thanks.

I am glad that you got it working.

In case you do future projects here is some more explanation about pin 15 and pin 8 (not pin 0, sorry my eyes are bad).

Pin 15 supplies power to the chip. Similar to electricity supplied to home appliances. Each applicance (ie chip) needs its own supply to operate correctly regardless of the other connections.

Simple analogy for sci.electronics.basics: Suppose I buy a new TV and a new stereo amplifier. I connect the TV audio output to the stereo audio input. I plug the TV into the wall outlet and turn it on and hit the "on" switch of the stereo ..... no sound comes out of the stereo. I forgot to plug the stereo into a wall outlet. Both the TV and the stereo amplifier needs power to operate.

The schematic with the single 74HC123 has both the A and B circuits powered from pin 15 of the single chip. If you use separate 74HC123 chips then you must provide power to each of the chips. ie. Both 74HC123 chips must have their pin 15 attached to the +5V power supply. (ditto for the ground on pin 8).

Be aware that sometimes (infrequently) schematics omit the power supply pins and ground pins for chips assuming that readers of the schematic know that chips need power.

Sorry for my bad eyesight. As other posters have noted I mis-read the pin assignments from the schematic.

My apologies.

However the discussion of providing power to chips is still valid if you omit my pin specific comments.

I was trying to make a general comment about powwer to chips. A newbie looking at a schematic of multi-function chips may be lead to believe that two separate chips are required since two rectangles are drawn on the diagram. If no power is supplied to the second chip then the circuit will not work. Generally speaking if the power supply pin of a chip is not connected then that chip will not function.

Thanks again. Yes I was a little confused by the 15 / 16 Input power I thought there was some confustion. Im glad its cleared up. I get confused wth fuzzy logic.

Regards

Even

Again thanks for filling in more data. And thanks I will look at the diodes addition. Regards