Mesa 7i76e - 3.3v Step Output - Delta A3

Hi together,
on my second LinuxCNC build now. A good step from No-Name Steppers to a Delta A3 Servo System.
During the selection of the system I must have missed one difference from the Delta A2 to the A3 Servo models.

The A3 series seems to only accept 3,3V differential signals, compared to the A2 - which accepted 5V for it.

What would be the best way to output 3,3V signals?
- Using Bidirectional Logic Level Shifters like a TXB0108
- Access 3,3V from the FPGA directly - could not come up with a way about the how jet


Thanks in advance
Friedrich

Ritterchen wrote:

Hi together,
The A3 series seems to only accept 3,3V differential signals, compared to the A2 - which accepted 5V for it.

I think the subject of the note is the information that 24V communication should not be used. Not that you don't use a normal 5V line-driver output.
A standard line-driver output controller (RS-422) under the load of the receiver usually gives a differential voltage of just around 3.5V.

However, if you want to limit the input current to the driver input for safety, use a limiting resistor. According to the picture in the ASDA-A3 Series User Manual, 2 pieces of 51R resistors in series are used at the input. This would mean a current of around 24mA at a differential voltage of 3.5V and a drop 1V on LED in optocoupler. Reliable safe current is 20mA. So if you want to use a 5V line driver, use a limiting resistor of approx 100R at the driver input.

You don't need much, the differential output resistance of the 7I76E is already about 80 Ohms, adding an external 100 Ohm resistor would give you about 14 mA.

Thank you for all the replies here!
I am not that deep into the electronics (yet) - the hint with the output resistance together with the reference helped me a lot.

With a 100ohm resistor it runs at 12mA, without at 17mA looks good enough to use it without the resistors in between.

Thanks again