Confusion regarding HC245 and HC541, HC vs HCT  - Page 1
Forum section: electronics / beginners

author: John Celo
author_rank: Regular Contributor
timestamp: December 09, 2024, 05:18:03 am
content: I'm looking at HC541 (unidirectional tristate-buffer/driver) and HC245 (bidirectional tristate buffer).HC541 is specifically stated as a "driver". How exactly is HC541 a "line-driver" and HC245 not? They both seem to have similar performance characteristics. Main difference being bidirectionality of HC245.I want to stock up and use HCT variant of 245 as a generic part in parts bin, which could also be used as a level shifter (3.3V->5V in output direction, because HCT part recognizes 2V as high input, and HC part needs 3.5V as high input) as well as fullfill all the functions of HC541 would.HCT version being bit slower (about 2x slower max switching speed, but still in 10+ mHz range?) and costing bit more than HC versions.HC245 are generally speaking also significantly cheaper than HC541 parts (LCSC pricing), so 245s also seem the way to go even if you don't require bidirectionality.HCT245 seems like the choice to go to unless very fast switching speeds (on a bus) are required.Am I understanding this correctly?

author: David Hess
author_rank: Super Contributor
timestamp: December 09, 2024, 09:02:01 am
content: Quote from: John Celo on December 09, 2024, 05:18:03 amThey are just calling it a driver based on application. The drive capability of both integrated circuits is identical.

author: CaptDon
author_rank: Super Contributor
timestamp: December 09, 2024, 01:45:19 pm
content: The 541 being stated as a driver is just as David said, the method in which it is typically used. Inputs that may consume one unit load with outputs that can drive 20 or 40 unit loads. Since it is uni-directional, a one way street it could drive LED's used as process indicators or status indicators. The 245 is bi-directional and termed as a 'buffer' because it has current gain in both directions therefore a CPU is able to drive an address or data buss and a single RAM replying back is also able to drive a heavily loaded buss by talking through a buffer. It's just a word usage convention when an engineer wants to search a data base specifically for a driver or a buffer. I have often seen 245's connected as a one way street in which case it would be considered a driver. Probably done to reduce the variety of part numbers in a project and maybe even get volume discount using more of one particular part number.

author: cte
author_rank: Regular Contributor
timestamp: December 09, 2024, 02:27:50 pm
content: Regarding HC and HCT, those documents might be helpful:Logic Guide (Texas Instruments)An Introduction to and Comparison of 74HCT TTL Compatible CMOS Logic (Fairchild / ONsemi)

author: David Hess
author_rank: Super Contributor
timestamp: December 09, 2024, 06:40:29 pm
content: It is confusing because there are logic chips labeled drivers which do have increase output drive and can directly drive a low impedance transmission line. The 74HC541 is not one of these chips.

author: tooki
author_rank: Super Contributor
timestamp: April 26, 2025, 11:00:59 am
content: Quote from: cte on December 09, 2024, 02:27:50 pmClickable links:Logic Guide (Texas Instruments)An Introduction to and Comparison of 74HCT TTL Compatible CMOS Logic (Fairchild / ONsemi)

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 26, 2025, 01:21:40 pm
content: Quote from: edavid on December 09, 2024, 07:39:07 pmI just had a brief look at a datasheet from an old Philips datasheet (before NXP, dated December 1990) of an 74HC/HCT541 and in it's "features" it mentions:QuoteI'm not entirely sure how far compatibility of such features goes between different chip manufacturers. I've heard several stories of swapping a TTL IC with an IC from another manufacturer made a circuit unreliable, or not work at all. (But bad designers also rely on obscure features, so I don't know what was really happening).

author: jwet
author_rank: Frequent Contributor
timestamp: April 26, 2025, 07:27:04 pm
content: The drive stuff has been covered- what I didn't see mentioned was the HC/HCT Icc variation. HCT do have the lower Vin high threshold but draw about 1/2 mA per pin doing this. Kind of buried in fine print. HCMOS draws nearly no static current with input full hi or low. With TTL like inputs on HCT, they burn a bit.

author: tooki
author_rank: Super Contributor
timestamp: April 28, 2025, 05:28:29 pm
content: Quote from: jwet on April 26, 2025, 07:27:04 pmDid you read the Fairchild/onsemi document? It goes into a lot of detail of when and why you’d use HCT vs HC.