74S vs 74LS ICs UND EE201L

Question

I am starting my EE degree. I must purchase a myDAQ and a kit. The kit lists many logic ICs and I have most of them, but some of what I have are 74S instead of 74LS series.

Will the extra power draw on the 74S ICs cause issues with the myDAQ?
or do I need to bite the bullet and buy the parts?

(on digikey, I am NOT paying their markup on their stupid kit).

The parts kit consists of things like or, and, xor etc.... gates and can be found here: https://www.studica.com/us/en/NI-Hardware-Only/und-student-ni-mydaq-bundle-ee202-electric-lab/796087.html

Edit In case anyone else runs into this in the future, it turns out not to matter for the UND EE201L course. We had minimal fan-out and most of the labs were just to use specified inputs to create specified outputs and make a led blink/turn on/ turn off. There were no high speed requirements and I wasn't driving high current loads, it was just an intro to logic gates and how to build circuits with them so I doubt it would have been an issue to use the 74S parts instead of the 74LS parts. It was a good learning experience though and the myDAQ is a decent tool to have in my tool box since I don't own a quality function generator (yet). Just in case, I ran power from my bench top power supply instead of the myDAQ

Answer 1

Good question. Here is a high level comparison of current consumption.

Note the difference in the values of \$I_{OL}\$ of two devices (for example). 74S has \$I_{OL}\$ of \$20 mA\$ where as 74S has \$I_{OL}\$ of \$8mA\$. The drive ability is different. If you are only doing this for a low speed simple circuits this will be okay but if there are experiments planned considering high drive strength of 74S, you might have to use a buffer.

I would suggest to really go ahead with the S version alone. the LS version are low power and better for a product design, but the S version will do just fine for basic circuits, which is planned. This is a hands on kit and hence i strongly recommend to use what you have and buy only those which you really need in future. IF there are circuits, which are pushing the LS version to their limits, you can handle it with buffers or buy.

I will compare the differences in the performance of the two devices SN74S04 and SN74LS04 and how and where it matters:

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• Current Consumption SN74LS04 consumes a maximum of \$6.6 mA\$ when outputting a logic zero compared to \$54 mA\$ max of SN74S04. It also means that, the internal temperature of the component will be higher. It also adds to overall current consumption of the product. It reduces battery life (wastes power) or demands higher capacity battery.

• Ability of FANOUT the LS part has about \$8 mA\$ of \$I_{OL}\$ and \$0.4 mA\$ of \$I_{IL}\$. It means, when connected in a system, an LS IC can support upto \$ 8/0.4 = 20\$ devices. The S part on the other hand has \$I_{OL}\$ of \$20 mA\$ but \$I_{IL}\$of \$2 mA\$. Hence, the part can drive upto \$10\$ devices theoretically. Hence, LS part is better when there are multiple inputs driven by the single output pin. The fanout may demand perhaps a new buffer to support the fanout needed in S family when compared to LS family, adding to cost and size on the PCB.

• Below is a simple comparison from http://www.ti.com/lit/sg/sdyu001ab/sdyu001ab.pdf which compares the applications of LS and S parts from TI (in general).

Answer 2

I would expect all sorts of unanticipated problems substituting an S part for an LS.

Enough strange things happen in student labs as it is -- you really don't want to be that guy with the non-standard kit.

I used to be a lab TA, a long time ago. If I had a student that was having trouble, and had non-standard parts, then unless I was in a good mood and they had an absolutely top-notch sob story about why they had non-standard parts (and "I was offended by the price" is not adequate), my troubleshooting efforts would be confined to "get the right parts, and let me know if you still have trouble". If I were a prof in such a situation, I'd back up my TA.

Answer 3

You might have to pay attention to fanout if you're driving 74S with 74LS or CMOS outputs. It's only about 2 from 74LS (check the datasheets)- meaning one 74LS output can only drive 2 74S inputs (in general) without potentially compromising noise margin.

The timing is also different (quite a bit- maybe 3x- faster) so there might be other issues, depending on the circuit.

Answer 4

74S chips run warm, drawing considerably more DC quiescent power from your MyDAQ +5v supply than 74LS chips. For example, a 74S00 dissipates 19mW where a 74LS00 dissipates 2mW.
You should pay attention to the 500mW limit total power available from all three supplies (5v, +15v, -15v)...both 74S & 74LS chips don't like a DC supply that sags or glitches. A breadboard full of 74S chips may stress MyDAQ's DC supply.
Note that a 74LS output pin can drive up to four 74S input pins, compared to ten 74LS input pins - likely not a serious problem for simple logic circuits.

A cryptic note in the MyDAQ user manual (avoiding a proper description of logic voltage levels and current levels) describes MyDAQ's somewhat odd digital I/O. It seems odd that no +3.3V DC supply is offered but TTL compatibility seems possible:

The digital I/O lines are 3.3 V LVTTL and are tolerant to 5 V inputs. The digital output is not compatible with 5 V CMOS logic levels.

Since you'll likely be wiring 14-pin DIP chips on a breadboard, some careful attention should be paid to keeping high-speed logic transitions tamed, as Spehro has suggested. This would include power-supply smoothing capacitors on every logic chip.

Answer 5

If I got this right, the myDAQ will provide "up to 500m Watts of Power". You can power three 74S04 with that (see Umar's answer for the details), but that's it.

if you really need/want to use the "S" components, you will most likely need to add your own power supply (for example, a USB power supply; you might get away with 1A, but they cost almost as much as 2A USB PSUs).

I did not check the prices for the required components, but I guess that the break-even point is at around saved 10 ICs.

Honestly, I'd say it's not worth the hassle to add an additional power supply..