74HCT245 vs 74BCT245 - Page 1
Forum section: electronics / beginners

author: GadgetUK164
author_rank: Contributor
timestamp: April 08, 2020, 05:26:44 pm
content: Has anyone here had experience with the 74BCT245 from Texas Instruments? I was sold the BCT245 as a HCT245, and I suspect there is a difference, which I am finding causes intermittant behaviour depending on temperature. I believe the part of the circuit I am working on needs HCT245, and these BCT245 are causing the issue I am seeing.I've had a look at the datasheets but cannot make much sense. The difference I've just noticed, just measuring output on multimeter of one of the data pins, it shows around 1v. Comparing to another board of the exact same type with HCT245s on it I see barely any DC voltage to speak of when measuing the same 2 pins in the same position (both boards carrying out the same activity to test).What are your thoughts, is there something very different about the BCT245? I am also considering they might be fakes and not even either BCT or HCT chips.EDIT: I just measured them when they were switched to high impedence. I meaure 1.4v on the BCT245s (which is very strange imo), and comparing to an identical board using HCTs in the same position, again in high impedence state, I measure 0.7v (this was a surprise too considering I expected it to be kinda floating). But there's a noticeable difference there, roughly double.

author: Benta
author_rank: Super Contributor
timestamp: April 08, 2020, 06:30:13 pm
content: You can't really measure voltage on three stated outputs.On paper, the BCT should be a drop-in for HCT, but with shorter delays and faster rise/fall times.I suspect there might be a difference in the pinouts, though.

author: GadgetUK164
author_rank: Contributor
timestamp: April 08, 2020, 07:19:42 pm
content: I think I've worked it out. Apparently BCT has TTL compatible inputs and outputs. HCT has CMOS compatible outputs.Going to swap these for some proper HCT chips.

author: Gyro
author_rank: Super Contributor
timestamp: April 08, 2020, 08:56:53 pm
content: Not quite, no.HChas CMOS compatible inputs (thresholds), HCT has TTL compatible inputs.As Benta says, BCT is faster.(EDIT: 74BC and 74BCT variants differ in exactly the same way. Theyallhave CMOS output swings btw.)

author: langwadt
author_rank: Super Contributor
timestamp: April 08, 2020, 10:10:18 pm
content: Quote from: Gyro on April 08, 2020, 08:56:53 pmHCT: V_OH @ -6mA/4.5V = min 3.84V typ 4.3VBCT: V_OH @ -3mA/4.5V = min 2.4V typ 3.3V2.4V at 4.5V Vcc is not CMOS swing

author: David Hess
author_rank: Super Contributor
timestamp: April 10, 2020, 10:29:12 am
content: HCT is HCMOS with TTL compatible input levels intended to replace standard bipolar TTL and derivatives like LS (low power schottky). It can be used in place of BCT TTL when higher speed and higher drive capability is not required.BCT is built on a BiCMOS process which I suspect means bipolar input and output structures with CMOS between them. It is faster than HCMOS and has enough drive capability to implement incident wave switching with 50 ohm transmission lines which means it can drive a 25 ohm impedance.So BCT is about 3 times faster than HCT and has about 8 times the drive capability of HCT. Some circuits could either either but it will depend on the details.