Unexpected output on TTL gate - Page 1
Forum section: electronics / beginners

author: netdudeuk
author_rank: Frequent Contributor
timestamp: July 17, 2017, 01:40:26 pm
content: HiI've got a 74LS32 where I don't see what I expect on one of the outputs.Here's a portion of the schematic.U8.2 has a low signal from another gate on pin 5 and a digital stream into pin 6 (Max voltage shown as per the scope). However, rather than seeing the same stream on the output pin 4, I see a steady 1.28V according to the scope.I don't see any shorts on pin 4.Why would this be ?Thanks

author: MK14
author_rank: Super Contributor
timestamp: July 17, 2017, 01:51:17 pm
content: I'd check the 74LS32'sPower pins (especially Vcc) is present/connected and at 5V, and ground pin.See if its output pin is shorted to another output, somewhere. Maybe (unlikely, as voltage would probably drop towards 0 when logic 0) even the base of a darlington transistor, with the base resistor missed/shorted out or something.Or possibly a faulty 74LS32.Disclaimer: Or 1,001 other possibilities.

author: netdudeuk
author_rank: Frequent Contributor
timestamp: July 17, 2017, 02:01:01 pm
content: I've measure 4.97v across the 74LS32, not shorts between the pins and I also have a decoupling capacitor across it.The device is a new one, which I used to replaced another new one.Thanks

author: MK14
author_rank: Super Contributor
timestamp: July 17, 2017, 02:04:05 pm
content: Quote from: netdudeuk on July 17, 2017, 02:01:01 pmThe short can be between its output pin, and another output, anywhere else in your bigger circuit. From any of the other IC's you have.Hopefully you are using good quality parts from a reliable source.

author: Nusa
author_rank: Super Contributor
timestamp: July 17, 2017, 02:08:52 pm
content: http://www.ti.com/lit/ds/symlink/sn74ls32.pdfSwap pins 4 and 6. Pin 6 is the output of the gate, pin 4 is an input.

author: albert22
author_rank: Regular Contributor
timestamp: July 17, 2017, 02:11:47 pm
content: Please check the pinout that you are using according to the datasheet you are using an input as output and the output connected as an input.

author: netdudeuk
author_rank: Frequent Contributor
timestamp: July 17, 2017, 02:13:17 pm
content: Quote from: albert22 on July 17, 2017, 02:11:47 pmI just came on here to share this thanks

author: netdudeuk
author_rank: Frequent Contributor
timestamp: July 17, 2017, 02:17:24 pm
content: Quote from: albert22 on July 17, 2017, 02:11:47 pmSo, yes, that was it thanks.

author: netdudeuk
author_rank: Frequent Contributor
timestamp: July 17, 2017, 02:18:46 pm
content: Quote from: Nusa on July 17, 2017, 02:08:52 pmYes, that was it thanks. I simple chip like a 74LS32 comes in different pinouts ? Hard to believe.

author: MK14
author_rank: Super Contributor
timestamp: July 17, 2017, 02:24:29 pm
content: Quote from: netdudeuk on July 17, 2017, 02:18:46 pmThe second picture you posted, seems to be the CMOS 4000 series of the same (OR) gates. Not a 74LS32.There is a web reference to your picture and a 74LS32. But it is probably a mistake.***MISTAKE*** here ==>>http://www.rshelectronics.co.uk/product.php?id_product=996But I'm not 100% sure.Vss and Vdd are NOT terms that bipolar TTL would normally use.

author: Nusa
author_rank: Super Contributor
timestamp: July 17, 2017, 02:27:55 pm
content: Quote from: netdudeuk on July 17, 2017, 02:18:46 pmThe 4071B Quad 2-input OR gate package has the other pin-out. That's presumably where the second picture came from.

author: MK14
author_rank: Super Contributor
timestamp: July 17, 2017, 02:51:19 pm
content: I've emailed where I think you bought it from, and told them their pinout seems to be wrong.