74LS794 question

[Wawavoun]

Hi All,

I expect to use a 74LS794 on a Motorola 6809 bus.

I hope to use it as a memory and store and read values into it. I have a /SEL signal which goes low when the LS794 address is selected.

The LS794 has two controls signals :
OE which enable the readback (outputs are connected to inputs when low)
CK which trigger at low to high transition the register so the inputs are stored into the latches and directly send to output.

Of course my bus has standard 6809 E and R/-W signals...

What is the best way to connect the controls signals with minimal glue logic...

I have what I think is a solution but I found it heavy...

Thanks and regards.
Philippe

[Ian.M]

Logic Friday^* reckons this is equivalent, which uses only 3/4 of a 74LS02. However the /OE signal has more propagation delay.


* See: https://www.eevblog.com/forum/projects/74-series-logic-design-equation-optimiser/msg3170938/#msg3170938

[Benta]

Why choose a part that's been obsolete for 20+ years, was never popular and the chances of finding NOS are close to zero?
The readback function that you'd like can be done with a standard 74HCT-type register with 3-state outputs.
The decoding for the latch doesn't get much simpler than three gates.

[MathWizard]

Hmm I see the 74 series doesn't like low temperatures like the freezing point of water 0C. IDK the average working temp of most parts, but yeah that's something to watch out for up here in the north.

But aren't the HC parts the older slower versions, of stuff like LS parts ? Or Benta do you mean the part itself is not as good for the application ?

[kjpye]

Hmm I see the 74 series doesn't like low temperatures like the freezing point of water 0C. IDK the average working temp of most parts, but yeah that's something to watch out for up here in the north.

But aren't the HC parts the older slower versions, of stuff like LS parts ? Or Benta do you mean the part itself is not as good for the application ?

If you want a wider temperature range you want the 54 series devices.

No, HC devices are newer than LS. First there was the 74 series, then 74H (high speed) and 74L (low power). After that came 74S (Schottky) and 74LS (low-power Schottky—which became the go-to family). After that there were faster families like 74ALS and 74F and others. In parallel with some of the later families, a cmos version family (74C) came along, and then 74HC (high-speed Cmos) and 74HCT (with TTL signal levels).