As far as I can tell, there are AM26LV32 chips on the SnapAMP to implement the differential encoder channels.
Do you use any terminating resistors for the differential signals?
I want to make a daughter board for the KFlop to turn some of those single ended encoder channels into differential ones, and would like to duplicate the SnapAMP implementation....

Thanks for the info!!

Is there any reason, I can't just use the Z,Z* signal from the differential encoder and pump it directly into the opto_pos/opto_neg inputs on the SnapAMP? Seems like a nice way to keep the differential signal, well, differential.

Can you explain why a standard line driver would not be able to push 5mA? The AM26C31 line driver you cited earlier will push +-20mA all day, it seems.

If the line driver is supplied with 5V, why would the output be anything but that?

I think the SnapAMP uses TLP281-4 chips. The diode drop is highly dependent on both temperature and current (steep curves), but the actual effect is small (small range). So the forward drop can be estimated as 1V.

This
http://www.dynomotion.com/Help/SchematicsSnap/SnapAmpConnectors.htm
Specifies a 470 ohm limiting resistor.

5V - (about 1V diode drop) = 4V. And by V=IR we get I = 4/470 = 8.5 mA (which is much more than the quoted 6mA.)

It is hard to know how this will behave in reality, because the documentation shows the FPGAs opto input going to ground through the coupler. I suspect that there is a pullup resistor in play somewhere, but without knowing for sure what that size is, I can't easily guess as to what the minimum current is to drive the logic down.

The current transfer ratio of the optocoupler looks to be unity or better at 1mA. 1mA seems too high of a current for a reasonable pullup. So I suspect that even at 1mA drive the opto can easily deal with the pullup.

Thus, I expect that the optocoupled input may work at a milliamp or two. That would only require 0.5-1V over the diode drop, or a total of roughly 1.5-2V. Even if the line driver were to run at 4V, I don't see a problem there.

Am I doing something wrong?

Thanks for taking the time to read this!

The SnapAmp differential inputs do not have any termination.  I think the recommended termination is 120ohms depending on the driver and the cable type.  We elected not to add the termination for cases where the encoders could not drive the 120ohm load or the signals needed to go to multiple inputs.  For Kanalog we decided to add the 470ohm termination as a compromise. 

HTH

Regards

TK

 

If it's anything like the Kanalog encoder inputs, then there will be 470ohm terminating resistors.

You could always just measure across an encoder input with a multimeter to chek.

I was going to suggest I've just made some driver boards using the corresponding AM26C31 driver chip, however having just checked the datasheets, the differential channels and single ended signals are all swapped around, so you can't just swap chips.

Moray

On Sat, Nov 12, 2016 at 10:49 PM, homeloan@... [DynoMotion] <DynoMotion@yahoogroups.com> wrote:

 

As far as I can tell, there are AM26LV32 chips on the SnapAMP to implement the differential encoder channels.
Do you use any terminating resistors for the differential signals?
I want to make a daughter board for the KFlop to turn some of those single ended encoder channels into differential ones, and would like to duplicate the SnapAMP implementation....

I think Moray's calculations are correct.

The Optos on SnapAmp have a guaranteed CTR of 50-600% at If = 5ma.  But might be less at If=1ma and worst case temperature.

The output of the Opto connects to the FPGA and utilizes the FPGA internal "weak" pull-up option which has a specification of 0.25ma pull up maximum.

We conservatively spec the inputs to work when switched (ie mechanical switch) to +5V +/-10%. 

HTH

Regards

TK

 

There is a volt drop between the input and output voltages.

I know having tested my driver boards, the output is showing as 4.5V with no load, but having just checked the datasheet, it specifies the typical High output as 3.2v (minimum of 2.2/2.4 depending on chip), and a typical low voltage of 0.2V (possibly as high as 0.4V).

That means the typical voltage should be 3V, but could potentially be as low as 2V.

Off course, it depends on what source you're using. There are higher voltage line drivers available, but it all depends on what you're trying to achieve.

Going by the fact a standard opto input will work, I'm guessing this isn't for an encoder input?

In which case, you could probably get away with a just a simple transistor setup to amplify the signal the a bit, combined with a reasonably strong pull-down resistor near the transistor to ensure a reasonable amount of current has to flow to switch the transistor.

On Sun, Nov 13, 2016 at 12:07 PM, homeloan@... [DynoMotion] <DynoMotion@yahoogroups.com> wrote:

 

Can you explain why a standard line driver would not be able to push 5mA? The AM26C31 line driver you cited earlier will push +-20mA all day, it seems.

If the line driver is supplied with 5V, why would the output be anything but that?

I think the SnapAMP uses TLP281-4 chips. The diode drop is highly dependent on both temperature and current (steep curves), but the actual effect is small (small range). So the forward drop can be estimated as 1V.

This
http://www.dynomotion.com/ Help/SchematicsSnap/ SnapAmpConnectors.htm
Specifies a 470 ohm limiting resistor.

5V - (about 1V diode drop) = 4V. And by V=IR we get I = 4/470 = 8.5 mA (which is much more than the quoted 6mA.)

It is hard to know how this will behave in reality, because the documentation shows the FPGAs opto input going to ground through the coupler. I suspect that there is a pullup resistor in play somewhere, but without knowing for sure what that size is, I can't easily guess as to what the minimum current is to drive the logic down.

The current transfer ratio of the optocoupler looks to be unity or better at 1mA. 1mA seems too high of a current for a reasonable pullup. So I suspect that even at 1mA drive the opto can easily deal with the pullup.

Thus, I expect that the optocoupled input may work at a milliamp or two. That would only require 0.5-1V over the diode drop, or a total of roughly 1.5-2V. Even if the line driver were to run at 4V, I don't see a problem there.

Am I doing something wrong?

Thanks for taking the time to read this!

---In DynoMotion@yahoogroups.com, <moray.cuthill@...> wrote :

I'd say a line driven source won't be enough to power the opto.

Looking at the data on the help page, the SnapAmp optos need 6mA at 5V, and if it's only a 5V differential source, it's more likely to only give 4V.

Moray

On Sun, Nov 13, 2016 at 3:55 AM, homeloan@... [DynoMotion] <DynoMotion@yahoogroups.com> wrote:

 

Is there any reason, I can't just use the Z,Z* signal from the differential encoder and pump it directly into the opto_pos/opto_neg inputs on the SnapAMP? Seems like a nice way to keep the differential signal, well, differential.

Thanks for pointing that out. I did not catch that the typical line driver output voltage is so much lower than the input voltage. The AM26C31 specifies a minimum of 2V, it seems...

You say that you are guessing that this is not for an encoder input, but that is exactly what I was planning to use it for. I was hoping that the differential Z signal could drive the opto for the index.

I guess I will just use another AM26LV32, and then pump that signal into a GPIO..

Thanks for the help!

Thanks for the info!

I think you guys are right. I will just use a AM26LV32 to decode the Z puls