Open-collector vs. standard TTL question (ans'd)

Thread Starter

metermannd

Joined Oct 25, 2020
211
So I'm monkeying around with another part of a schematic where it used two sets of NAND gates - a standard 74LS00 and the open-collector version, the 74LS03.

After a bit of feature cleanup, I have three unused 74LS03 gates and one unused 74LS00 gate, so I've a mind to keep the 74LS00 if I can.

The one remaining 74LS03 gate fed a gated clock signal to the NVRAM chips, but it now goes off-board and passes through a buffer on another board to serve as the master clock for that board.

Also, that 74LS03 gate currently has a pull-up resistor on it.

As I understand it, the main reason for using open-collector TTL is for interfacing with non-TTL loads, right?

Therefore, if running straight TTL all the way through now, there's no longer any reason to keep the remaining '03 gate, and I can just swap for the one unused '00 gate and then drop the '03 completely, right? Would I need to keep the pull-up if I swap that gate to use the '00?
 

MrChips

Joined Oct 2, 2009
24,222
Open collector output can be used to switch loads at voltages other than 5V.

It is also used to create wired-OR logic, in other words, creating an OR logic function (for negative logic) without requiring additional gates. For example, suppose you have two signals that are allowed to pull an interrupt request line low. To do this with logic gates you would need an OR gate with inverting inputs and inverting output (i.e. and AND gate). Now suppose you wanted to add another signal with the same interrupt capability. You would need more gates. With open collector logic, no gates are need to create the OR function.
 

Thread Starter

metermannd

Joined Oct 25, 2020
211
sghioto: Perfect, thank you!

dl324: oh? I wasn't aware of that... will check that out. Will be ordering parts for my second board this week, will keep that in mind.
 

dl324

Joined Mar 30, 2015
13,128
dl324: oh? I wasn't aware of that... will check that out. Will be ordering parts for my second board this week, will keep that in mind.
If you want the ability to mix and match, 74HCT has TTL compatible inputs and has sufficient output drive strength to drive multiple 74LS inputs. 74HC/HCT buffers don't have as much drive capability as TTL; about half as much IIRC.

EDIT: Here's a table from Philips Semiconductors; my recollection was correct:
clipimage.jpg
HC and HCT output capabilities are the same.

EDIT: Added attribution for the table.
 
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