Is it possible to directly link an 84+ CE to an older calculator with the old link port connector? Or is the only way to transfer between them is by using a PC or Mac as an intermediary?

This is not a big deal in my case as I have both sets of cables to link my calculators to my Mac here. I’m mainly just curious about what can and can’t be done.

dennis

The only supported linking on the CE models is Direct USB, which requires USB on both ends. So you should be able to communicate with other 84+ models, but not 89T (which has USB but doesn't speak a compatible protocol) or anything else (since the rest lack USB).

TL;DR: use a computer, which will provide backups of content, if nothing else

On the USB port, the 84+ and its derivatives (84+SE, 84+Pocket.fr, 84+CSE, 82A) use the same high-level protocol, officially named "CARS", as the TI-eZ80 series (83PCE / 83PCEEP, 84+CE, 84+CE-T / 84+CE-T PE, 82AEP). However, the newer models use larger buffer sizes, some more file types, etc.
So at best, you might be able to transfer a subset of file types and file sizes through direct connection between these classes of models; two successive transfers with the computer as an intermediary fix the buffer size issues.
The 89T also speaks CARS on the lower level, but when TI released the 84+ series a year later, they used minor protocol differences in nearly all packet types, causing near-complete effective protocol incompatibility between the 89T and the 84+ series (and a decade later the TI-eZ80 series). Of course, since there's complete incompatibility on a file format level as well, it made some sense not to make these models willing to talk to each other.

A mini-A male <-> A female USB OTG cable makes it possible to mechanically plug the SilverLink onto the newer class of models, but:
* I don't know whether the USB port provides enough power to supply the SilverLink adequately;
* third-party ASM programs would be needed to deal with the SilverLink at all, AFAIK;
* third-party ASM programs are also required for implementing the variants of the "DBUS" protocol used by the older models on their 2.5mm stereo proprietary jack port.

I tested this out because I was curious but it doesn't look like it works. I connected my Rev M CE to a TI-83. I wonder if it would be possible to write a program using the new USB libraries in the CE C toolchain to connect?

To begin with, you should use the USB libraries to configure the calculator's USB controller + stack in host mode, and try and get the descriptors of the SilverLink. If that works, you can proceed iteratively

Sadly, the 83 doesn't support the is_ready or get_version DBUS command sequence, but you could use the send_key or get_memfree sequences.
For testing purposes, one of the quickest ways to obtain packet sequences is using TILP and libticables' packet dumps (in ~/.ticables).