I am trying to understand how this data interface works.

It's supposed to:

• translate (read) "OT" current difference into logical OUT (<10mA = 0, >17mA = 1)
• send logical IN to "OT" by varying the voltage across OT (<7V = 0, >15V = 1)

The voltage across OT terminals is around 18V and the current ~5..9mA when idle.

Specifically I don't understand how current sensing works in this circuit. Does D5, D6 or both play a role?

^{Note: the interface is for an OpenTherm 2.0 master, though I hope I have provided all the relevant details about the interface in the question itself such that any further knowledge of OpenTherm is not required to answer the question (if it is, please comment about what exactly is missing and I will provide it).}

I am trying to understand how this data interface works.

It's supposed to:

• translate (read) "OT" current difference into logical OUT (<10mA = 0, >17mA = 1)
• send logical IN to "OT" by varying the voltage across OT (<7V = 0, >15V = 1)

The voltage across OT terminals is around 18V and the current ~5..9mA when idle.

Specifically I don't understand how current sensing works in this circuit. Does D5, D6 or both play a role?

^{Note: the interface is for an OpenTherm 2.0 master, though I hope I have provided all the relevant details about the interface in the question itself such that any further knowledge of OpenTherm is not required to answer the question (if it is, please comment about what exactly is missing and I will provide it).}

I am trying to understand how this OpenTherm interface works.

It's supposed to:

• translate (read) OT current difference into logical OUT (<10mA = 0, >17mA = 1)
• send logical IN to OT by varying the voltage across OT (<7V = 0, >15V = 1)

The voltage across OT terminals is around 18V and current ~5..9mA when idle.

Specifically I don't understand how current sensing works in this circuit. Does D5, D6 or both play a role?

I am trying to understand how this data interface works.

It's supposed to:

• translate (read) "OT" current difference into logical OUT (<10mA = 0, >17mA = 1)
• send logical IN to "OT" by varying the voltage across OT (<7V = 0, >15V = 1)

The voltage across OT terminals is around 18V and the current ~5..9mA when idle.

Specifically I don't understand how current sensing works in this circuit. Does D5, D6 or both play a role?

^{Note: the interface is for an OpenTherm 2.0 master, though I hope I have provided all the relevant details about the interface in the question itself such that any further knowledge of OpenTherm is not required to answer the question (if it is, please comment about what exactly is missing and I will provide it).}