What are the available solutions for a 8-input OR function that is 74xx compatible?

Question

I need to implement an 8-input OR function that is compatible to IC chips of the 7400 series.

I already found a few solutions, but none of them is perfect. A perfect solution would be an OR-gate chip of the 7400 series with 8 inputs and a reasonably low propagation delay (<30ns).

The only 74xx OR-gate with 8 inputs is the 744078 (OR/NOR multifunction). The only versions of it that I could find in stores was the CD4078BE by Texas Instruments, but unfortunately it is horribly slow (300ns @ 5V).

Another solution is to use a 8-input inverter with open collector like the SN74ALS762N and then simply connect the outputs to an inverter. The chip is also quite fast, but unfortunately not available in the store of my choice.

Yet another solution is to use two 4-input NOR-gates like the CD74HC4002E combined with a NAND-gate.

I could also invert the inputs and then use a 8-input NAND-gate.

Are there any better solutions that I'm not aware of? Are there other fast 8-input OR, NOR or buffer/inverter gates (with open collector) than the ones I mentioned?

UPDATE: I found another chip, SN74BCT756N, an open collector inverter with 2 x 4 inputs. For whatever reason, the 74756 and 74757 are not listed in Wikipedia's "List of 7400 series integrated circuits".

Answer 1

OR is the same as AND in negative logic, so...

74LS688 8 bit magnitude comparator. With all Q inputs set low the NOT P=Q output will be low only when all P inputs are also low. That means if any P input is high the output will be high.

As a bonus it also has an active low 'enable' input, so it could even be used as a 9 input OR gate!

Maximum propagation delay is 23ns.

Answer 2

You seem to have covered most of the obvious ones, but you are limited by the 5V supply and whatever you mean by compatible with 7400 series (which could require the thresholds be TTL compatible).

If I assume the inputs have to be true TTL compatible, I would suggest a 74ACT02 quad NOR gate (6.5ns) and 1/2 a 74LV20 4-input NAND gate (6ns and 12mA drive) so total 12.5ns, which is well within your spec.

Generally the gates with 4 or fewer inputs seem to be more popular, so the range of types and availability tends to be higher. Of course you could use a CPLD, which is where a lot of the logic gate market has disappeared.

Answer 3

You looked at diverse stuff like 8 x open-drain buffer, so just to cover every avenue...

You could use two 6-pin BAT54CV Dual Common Cathode diodes driving a 4K7 pull-down into your next stage. It depends on the logic family you already have as to how suitable this is. With 74HC/HCT, it's fine but into and out of 74LS isn't possible. The driven LS gate would need a much lower pull-down resistor for it to detect a logic low and an LS output hasn't enough current to drive that resistor logic high

You could also use a 5 V PLD, such a 15 ns Atmel ATF750C, but that's probably a lot more aggravation than two stages of fast 74xx, unless you can mop up other stuff there, too.

Answer 4

The 74xx138 data selector can do 5-bits of OR (inputs G2A, G2B, A0, A1, A2, and output Y0). So, with an additional 3-in NOR connected to G1, it implements the right function. 74ACT138 datasheet here worst-case is 12 ns delay, so 74ALS27 (3-in NOR) would be fast enough to keep under 30 ns.

Answer 5

Bit of a slow answer, but there's a 74HC4078, worse case propagation delay (at 4.5V, -40ºC to +85ºC) of 24ns, but it won't accept a 2V logic 1.

Answer 6

8-input OR, NOR [unmentioned]?
Connect the expander pins of the expandable halves of two 7423 to get an 8-input, dual output NOR gate (with one strobe for each half), with two regular 4-input NORs (with strobe) to spare.