Replace HC by HCT, and LS by HCT - Page 1
Forum section: electronics / projects

author: luiHS
author_rank: Frequent Contributor
timestamp: May 23, 2019, 08:09:04 pm
content: Hello.I have a circuit that uses 7407 and 74HC374, in addition to other 74xx chips.The problem is that I want them in TSSOP format on a reel with 12mm wide tape for my Pick and Place Neoden4. Those two chips in particular I've only found them on 16mm tape.Looking for the 74HCT374 to find that it is on TSSOP 12mm tape, and I thought that I could replace the 7407 with a 74HC4050 that is also available on TSSOP 12mm wide tape. Can they be replaced without problems?On the other hand I have another circuit that uses 74LS chips, all of them only on 16mm tapes, even one on 24mm tape. Could I replace the 74LS by 74HCT? These I have found in TSSOP 12mm of tape width, I think that all 74LS can be replaced by 74HCT but I am not 100% sure.Regards

author: wraper
author_rank: Unknown
timestamp: May 23, 2019, 08:27:26 pm
content: LS can be replaced with HCT. That's the main reason HCT exists to begin with. However they have weaker output capability than LS and especially compared with plain 7400 series. Thus if you try to drive multiple 7400 inputs or too many LS inputs with 74HCT/HC, you likely will have an issue. When using HC series instead of TTL you need to take care if logic high on the inputs have enough voltage.https://www.onsemi.com/pub/Collateral/AN-368.pdf.pdf

author: wraper
author_rank: Unknown
timestamp: May 23, 2019, 08:36:53 pm
content: Also if circuit is poorly designed and depends on propagation delay a lot, you may have issues as well.

author: luiHS
author_rank: Frequent Contributor
timestamp: May 23, 2019, 08:41:59 pm
content: Thanks.Do you know if can I replace HC by HCT ?

author: wraper
author_rank: Unknown
timestamp: May 23, 2019, 08:45:43 pm
content: Quote from: luiHS on May 23, 2019, 08:41:59 pmYou usually can do this just fine. The downside would be reduced noise immunity due to lower logic high level. However I would expect the same packaging options available as long as HC/HCT come from the same manufacturer.

author: duak
author_rank: Super Contributor
timestamp: May 23, 2019, 10:34:47 pm
content: Luis, 74HC4050 has a different number of pins and pinout and is not open collector as is the 7407. It also has a different input voltage threshold as well as lower maximum output voltage and current . It may work in some circuits but there might be better choices. What circuit do you wish to use it for?

author: luiHS
author_rank: Frequent Contributor
timestamp: May 23, 2019, 11:16:24 pm
content: Quote from: duak on May 23, 2019, 10:34:47 pmThank you.I know that the pinout is different, both are non-inverting buffer, I thought I could use a single reference in two circuits redesigning the circuit that now install a 7407.In a circuit I use the HEF4050 to convert logic levels from 5v to 3.3v for the input pins of a Kinetis MK66 microcontroller, these pins are not tolerant to 5v.The second circuit works now with a 7407, I can not find this chip in TSSOP on 12 mm tape. I thought I can replace it with the 74HC4050, to use this chip in both circuits. Attached diagram of this circuit, there are pullup resistors to the input and output.

author: wraper
author_rank: Unknown
timestamp: May 23, 2019, 11:20:11 pm
content: Forgot to mention, HC/HCT must not have any inputs left unconnected.

author: Zero999
author_rank: Super Contributor
timestamp: May 24, 2019, 08:17:37 am
content: The 74HC has different logic levels than plain 74 and 74LS. It's also incompatible with 3V logic, when operated from 5V.The 74HCT has the same logic levels as 74(LS). It's useful for shifting 3V logic up to 5V and interfacing 74(LS) with 74HC logic. The 74HCT244 can convert eight 74(LS) or 3V logic levels to 74HC 5V logic levels.The 74HCT has a operating voltage range of 4.5V to 5.5V and the 74HC 2V to 6V.The 74(LS)07 can't always be substituted with the 74HCT07, because the 74(LS) version's output can be connected to 15V via a pull-up resistor, but the 74HCT07 has a diode connected between its output and VDD, so it can't be connected to higher voltages, than its supply. In this case it doesn't matter, since the output is connected to 3V.

author: luiHS
author_rank: Frequent Contributor
timestamp: May 24, 2019, 02:37:40 pm
content: Quote from: Zero999 on May 24, 2019, 08:17:37 amOk, thanks.Do you know if I can replace the HC with HCT? The circuit works at 5 volts, in that aspect there is no problem.Right now it is the only doubt that I still have. I have a couple of circuits in which some chip references coincide. In one of the circuits are LS and in the other are HC, my idea would be to put in both HCT, I know I can replace the LS with HCT, but I do not know if I can replace HC with HCT.

author: wraper
author_rank: Unknown
timestamp: May 24, 2019, 02:57:29 pm
content: I would just replace everything with HC and don't leave any LS at all. If there are things which have TTL outputs and cannot be replaced or are powered from 3.3V, I would use HCT inputs connected to those. As I said, HCT instead of HC should work just fine. HCT is a compromise to use CMOS technology but leave it backward compatible with TTL (LS).

author: luiHS
author_rank: Frequent Contributor
timestamp: May 24, 2019, 03:02:33 pm
content: Quote from: wraper on May 24, 2019, 02:57:29 pmOk, thanks, I will try to replace all the LS and HC with HCT.I will also try to replace the 7407 with the 74HC4050, I hope it works. The 4050 I use it a lot, and in this circuit only one buffer of the 7407 is used. Having to use a new reference takes away space in the feeders of the Pick and Place, so I try to reduce the number of references.

author: duak
author_rank: Super Contributor
timestamp: May 24, 2019, 05:17:03 pm
content: luiHS, the 74HC4050 should work fine in this circuit. As others have said, the unused inputs of the 74HC4050 should be connected to GND. Also, since the 74HC4050 is not open collector, R1 is not needed.I see that a 74HC04 is also used on this board to generate RESET_Z80. You could also use two inverters in series to perform the same function as the 74HC07. If there are two free inverters in the existing part, you would not have to add a 74HC4050.In fact, the circuit could probably changed slightly to use only one free inverter.

author: luiHS
author_rank: Frequent Contributor
timestamp: May 24, 2019, 06:01:36 pm
content: Quote from: duak on May 24, 2019, 05:17:03 pmOk, thanks.Unfortunately, the 6 gates of the 74HC04 are in use in the circuit, I have to use an additional chip for that single gate of the 7407. I prefer to use the 74HC4050, because it is a chip that I use a lot, and so I do not use more feeders of the Pick and Place.