Hi All,

I have a need to mix and match logic families. Just want to make sure I am doing it right.

On my circuit input I have a AM26C32 RS-422 Differential Receiver. This was selected as it is the mating product to the line driver (AM26C31) in the field installed encoder. I believe this has TTL outputs.

The bulk of my circuit uses 74AC series CMOS logic. I need to connect the AM26C32 (Datasheet) to D-Type flip flops CD74AC175 (Datasheet). The concern is that the V_OH (3.8V min) of the AM26C32 is less than the V_IH of the CD74AC175 (3.85V max). I understand this is a common issue with TTL to CMOS and can be solved by installing pullup resistors (10k ohm seeming to be a common value) on the connection between the two chips. I had originally picked out an AM26LS32A which had a much lower V_OH (likely TTL?). I think the AM26C32 (likely CMOS?) might be directly compatible. I'm not great at reading the datasheets but I am getting better.

Here is a snip from my circuit simulation (better schematic capture coming soon). The outputs from the AM26C32 would be on the left (labeled A, B, C) and the flip flops are on the right. The 3 10k resistors pull up to 5VDC. Will this work?

(NOTE: The schematic shows AM26LS32A. I think an AM26C32 is a better selection, but would appreciate verification. I might not even need the pullups.)


On the output side, I am using a AM26C31 Differential Line Driver (Datasheet) to drive the signals off the board and into a motor drive. I picked this part because I know it will work with the servo drive encoder inputs. The inputs are connected to the outputs of D-type flip flops (the same CD74AC175 as discussed above). The flip flops have V_OL of 0.5V max and a V_OH of 3.7V min. This seems to play nicely with the AM26C31 V_IL of 0.8V min and V_IH of 2.0V max, so no special interface is needed. Am I correct?

 

It will work fine. The pull-ups aren't really needed as the VinH spec of the CD74AC175 is spec'd at Vcc of 5.5v. If you're running them at 5v, VinH will be less than 3.85v. Also the VoH of the AM26C31 is spec'd at a 6mA IoH current - you won't be drawing anything like that so the VoH will probably be 0.1v or more higher.

 

It will work fine. The pull-ups aren't really needed as the VinH spec of the CD74AC175 is spec'd at Vcc of 5.5v. If you're running them at 5v, VinH will be less than 3.85v. Also the VoH of the AM26C31 is spec'd at a 6mA IoH current - you won't be drawing anything like that so the VoH will probably be 0.1v or more higher.

Awesome, exactly what I was hoping to hear!

 

exactly what I was hoping to hear

It's what you wanted to hear, but it's wrong.

Your table of input/output voltages shows that you need pull-ups on the CMOS inputs. The items circled in red should be output specs.

A typical HIGH output from the TTL part is 2.7V. The typical HIGH input for the flip flop is 3.85V. At 5V, the CMOS HIGH level input voltage will be 0.7*5V=3.5V. You're 0.8V short of having a reliable design.

This excerpt is for CD4xxx, but it's still mostly applicable to 74AC:


TTL driving CMOS without a pull-up will sometimes work, but it isn't guaranteed. If you want your design to be robust, you need to use pull-up resistors on the CMOS inputs.

 

It's what you wanted to hear, but it's wrong.

The confusion is my fault. I described it in my comments but the picture is a bit old.

I originally selected the AM26LS32A receiver, however I later identified the AM26C32 as a better fit with a much higher output voltage.

It looks like the AM26C32 has a V_OH of 3.80V minimum at Vcc = 5V.

Updated:

 

I described it in my comments but the picture is a bit old.

It would be helpful if you took the time to be consistent. If schematics are posted, I don't pay much attention to any "word salad" provided with them unless something about the schematic is confusing.

It looks like the AM26C32 has a V_OH of 3.80V minimum at Vcc = 5V.

You're worrying about something that isn't an issue. CMOS driving CMOS at the same supply voltage will have the correct logic levels; as long as you follow fan-in/out specifications.

 

So we're both right, but my answer was righter

For a minute there I thought I was losing my marbles - but I used mcardoso's datasheet link, I'd not noticed the different part in the diagram

 

It would be helpful if you took the time to be consistent. If schematics are posted, I don't pay much attention to any "word salad" provided with them unless something about the schematic is confusing.

I did apologize...

You're worrying about something that isn't an issue. CMOS driving CMOS at the same supply voltage will have the correct logic levels; as long as you follow fan-in/out specifications.

Sounds good. Fan-out is only 1 chip so I am good.

So we're both right, but my answer was righter

For a minute there I thought I was losing my marbles - but I used mcardoso's datasheet link, I'd not noticed the different part in the diagram

Appreciate the help.