There is the old Apollo (later Garmin SL70) and more recently the GTX 35R / GTX 45R.
A transponder is rarely interacted with (even in the UK LARS system ) and it is trivial to incorporate an interface for it in some other product. And you have a load of connections to it which need to be run to the back of the panel.
In the DME world, the KN63 was quite common, with the KDI572 control panel. But with transponders, it seems that only with the G1000 that the remote ones became common.
The very nice L3 NGT9000R is a remote xpdr/tas which can be operated via an IFD or Garmin. You’d miss the standard MFD provided with a NGT9000, but it might be a good solution when you lack panel space.
These small (round or square) boxes (without the buttons for 0-7) are terrible. With their very limited height, there is nothing wrong with the standard Garmin transponders. And therefore, in most cases, it doesn’t make much sense to separate the control head and the “computer”.
Most existing panels already have a hole in them that fits a transponder. So while upgrading to Mode-S, Adsb, etc. there is little need to go remote. Plus the actual transponder technology is not really big so that a standard instrument depth behind the panel area the interface box needs anyways is perfectly fine to hold the entire transponder.
With “total panel makeovers” that include GTN-Navigators I have the impression there are a lot with remote transponders because there you do not have that hole in the first place and a GTN is a good interface to operate a transponder.If you upgrade your panel and have the choice between a GTN650 and a panel transponder or a GTN750 with a remote one, I’d always go for the 750…
Many of the legacy transponders did not have a communication interface, so very few of the first several generations of GPS receivers added software support to control a remote transponder. With glass panels or larger GPS/MFD mounted in the panel, panel space became more of a premium and interface support was more common for a remote transponder. My panel mount GTX330 can be controlled from my GTN750, even though I never use the capability. So most modern transponders can be controlled remotely on both the panel mount and remote transponder versions and modern GPS systems can provide the control. So it is more a question of panel space for the transponder.
If I had to remote one thing, it’s the COM panel. The display in a GTN is very self-explanatory, more so than the buttons.
I kept it separate when I did my panel for two reasons: