The FlexWire connector on the FLEX-1500 utilizes the version 2 standard (v2) which is slightly different than the v1 standard used on the FLEX-5000 and FLEX-3000 software defined radios. The major difference is that pin 3, which was either grounded, un-used or physically blocked in some FLEX-x000 models, is now utilized as a PTT OUT for keying transverters and amplifiers.
Pin 3 is now connected to an "open-drain" which will provide a closure to ground during transmit for a positive voltage source. If you use this output to directly drive a relay, you must add a diode across the relay to limit the inductive "kick" upon releasing the relay. This protection diode is NOT provided internal to the FLEX-1500 and must be supplied by the operator.
FlexWire v2 connector
Specifications for the Pin 3 (keying relay) interface are:
Maximum current: 0.25 Amps
Maximum open circuit voltage: +30 Volts (DC)
Maximum "ON" resistance: 1 Ohm
Also, pin 8 has a higher output voltage (13.8 VDC or the value of the external VDC input for the radio) than the Version 1 standard which is 5 VDC.
The FlexWire v2 connector is a standard DB-9 female connector
Pin 1 - Ground
Pin 2 - Transmit Audio Line Input
Pin 3 - PTT Output (Open Drain)
Pin 4 - PTT Input (Also I2C interrupt request input, if used)
Pin 5 - Ground
Pin 6 - I2C Bus, Serial Clock
Pin 7 - I2C Bus, Serial Data
Pin 8 - +13.8 Volts Out (Same as voltage into power connector)
Pin 9 - Receive Audio Line Output