Icom Radio Transmit Inhibiting: ICM_TXD

You asked, we inhibited!

Icom radios have long lacked a really useful feature, transmit inhibit. The feature is exactly as it sounds: the ability to externally inhibit the ability of a radio to transmit through some sort of contact closure.

Such a feature is very useful for two main audiences, those who participate in radiosport contesting, who need the ability to limit the number of radios on the air at a time, and those who wish to interlock their radios to protect other sequenced devices in applications such as EME, or other weak signal applications.

The majority of higher-end Icom radios (see the full list of supported radios below) support built-in transmit inhibiting. The mute is found on the ring of the 1/8 in “EXT KEYPAD” jack. When the ring is shorted to GND (sleeve), the radio inhibits. Yet, it is seriously flawed. It also inhibits the receiver, making it nearly useless for our application. The nice feature here is that when engaged, the audio stream completely mutes, and you know your transmit is inhibited. 

So we have to combine these two ideas.

If one only tries to inhibit the radio when the inhibit contacts are closed, AND when the radio actively tries to transmit, then we can use this feature to great advantage. In short, the radio is only actually inhibited when we are both trying to inhibit, and trying to transmit. This means that there is no muted received audio, coupled with perfect transmit inhibiting.

The ICM-TXD is a blackbox-type device that handles the logic of this interlocking. The box connects to the ACC2 and EXT KEYPAD jacks on supported radios, using the provided 1/8″ – 1/8″ and DB9 – DIN8 cables. The interlocking contact is an RCA connector, which interlocks the radio on contact closure.

Two status LEDs are provided on the front panel to indicate when the interlock device is activated. There is also a pass-through jack for the EXT KEYPAD line, so that a user may still use an external keypad of their choice.

Front Panel of the ICM_TXD with indicator LED, TXD-In and Keypad Passthrough.
Rearview of the unit, shown in its metal case.

The manual can be found here.