Hi James,
I am indebted to our resident EHS expert for the following information:
"
The 3.5mm socket is for the Plantronics Serial Bus (PSB). This is an open collector 5 volt bus. The socket tip is power (9 volt nominal, same as wall power supply), sleeve is ground, and ring is data.
The bus protocol is complex and users a pulse width modulation arrangement to indicate a logic zero or logic one. Different PSB peripherals are assigned different address, so it is not a trial task to interface with, but there is an EHS which I think would suit your needs.
The APA23 normally connects to an Alcatel phone. It has two cables coming off a connection ‘hub’, a 6 way modular plug and a 3.5mm jack plug.
The APA23 tells the phone to change state (active/inactive) by applying a short circuit between the tip and ring of the phones Transmit audio connection, so you could simply apply a ‘pulled up bias’ between the tip and sleeve of the 3.5mm plug (perhaps 1k ohm pulled up to 5 volts would be a good load). The hook toggle command to the phone will then be a pull down of the tip towards ground for 400ms.
The command from the phone back to the APA23 is carried on its 6 way modular plug (Looking into a 6 way modular socket with the tab down, pin 1 is to the left).
Pin 5 is ground and pin 1 is a voltage that needs to be high (5 volts) whenever the phone wants the headset link to be active.
So if you want the phone to tell the base to go active, take pin 1 high (and keep it high).
If you want the phone to tell the base to go inactive, take pin 1 low (and keep it low).
If you want the base unit to tell the phone to change state (either active to inactive or vice versa), press the button on the headset and the 3.5mm plug tip and ring will short together for 400ms.
Important note: When the base unit asks the phone to change state, the phone must then output the correct voltage on pin 1 that matches what the base is expecting, so if the base is inactive, and you press the headset button to take the phone active, pin 1 must respond by going high within about one second of the 400ms short.
The base audio is carried on the 3.5mm jack plug. Sleeve is ground, ring is Rx, and Tip is hook toggle/Tx.
FYI, pin 5 of the modular plug is ring alert. Each time you transition to pin 5 to 5 volts, you will get a single beep-beep-beep ring sequence in the headset."
Good Luck!
Lewis.