Re: TNC Quirks
One was a Baofeng, one was a Retevis. I'm only a little surprised since I have a Baofeng setup as a bi-directional iGate and it functions perfectly in that role.
I could go on and one here about the issues with many of these cheap chinese radios be it their slow and/or inconsistent key up times (requires a high TXDELAY), their slow or inconsistent TX to RX time, inconsistent receiver issues, poor transmission signal quality, etc. and ultimately... all of these issues changing from one radio to the next (little quality from one radio to the next off their production line). It sounds like you might have gotten a "good" radio but many other people get something less. You will be far better served buying an older used radio from a decent company than these radios when it comes to consistent packet operation.
I don't see a TNC firmware update on Kenwood's software update page, just the main body and the control head updates. Also, the release notes don't mention anything consequential. However, both my main body and control head are two software versions behind. I'll have to use my work computer to handle the update since Kenwood's update software only works on Windows.
I looked as well. If you look at the D710A pages, you can see some TNC firmware updates but I guess the D710G hasn't needed any updates yet.
The Mobilinkd style TNC is a TNC I built using Mobilinkd's schematics and instructions. It is an Atmel 328P communicating with an HC-05 bluetooth serial adapter exactly like the Mobilinkd. I have a few photos of building it here: https://joshuajayg.smugmug.com/Tech/Ham/TNC/
Gotcha. Those setups really are pretty impressive in how they do so much but you're still sacrificing 1200bps AFSK decode rates compared to say Direwolf, a Kantronics KPC3, UZ7HO soundmodem, etc. It's all a compromise though.. it get it and these Mobilinkd units aren't bad by any means.
Also, I haven't found that Direwolf has a front end that will emulate a normal TNC like BPQ's Simple KISS does. If you know of a way it would work, I'm willing to give it a try.
It all depends on what you're looking to do. Why do you want to connect to a "classic TNC" serial terminal interface? Are you looking to create connected mode sessions to remote stations? Are you looking to send unconnected (unproto) packets? You can do all of these with Linux's native AX.25 support and either included or third party packet applications with just a KISS TNC. BPQ32 can do all these things with a KISS TNC w/o the TNC2 UI interface, JNOS can natively do these things, Qtterm + Direwolf's AGW interface can do these things, etc. I'm personally biased to use LInpac + Linux's AX.25 stack as I maintain that code ( https://sourceforge.net/projects/linpac/ ).
But in reality, I don't know of a reason my D710G needs to receive an incoming connection request. I imagine I will always initiate the request FROM the 710.
There are packet communities out there that run KB2KB packet stations where people leave packet messages on remote TNCs be it KPC3s, PK232s, D710s, etc. It's a simple, old school yet effective interface. The D710 makes this one step easier as the radio and TNC are in one (more expensive) package. If you're willing to work a bit for it, buying say a Kenwood V71 and connecting it up to say a Raspberry Pi Zero will yield you a solution with a substantially better TNC and a lot more flexibility with what you can do with the setup. It can be all self-contained, it can offer packet services via Wifi or Bluetooth, etc. Sky is the limit.