I had a client that bought one of these, and I found out a few things:
The OTHER Tascam USB driver, usbusx2y, expects to see vendor ID the US1640/1800/2000/200/600 doesn't send -- newer units send vendor ID for TEAC, not Tascam (or the other way around, I have it all written down somewhere).
Parts of the command set TEAC lists as proprietary, i.e., nonstandard. But it is not an in-house product. Apparently the drivers now available and internal firmware are product of a software house in Germany.
(Again, I have it written down somewhere.) Software for the 1640, 600 & 2000 all reference other USB products that I believe use the same family of Atmel MCUs found in the US1800, so there appears to be a common library involved. I have noticed there has been progress with these other products as to Linux functionality. The Atrmel chip used in the us1800 appears to be the only (or nearly only) member of its family that does NOT have USB capability built-in.
All USA US1800s have been shipped with the latest firmware version. This is NOT true for the Japan market -- there is (was) a firmware updater for early v.1.0 domestic units on tascam.co.jp. I have a copy of this file, and assume it has an image of the present firmware on it. I am not an Atmel programmer. A gentleman in Japan appears to be the maintainer of the usbusx2y files. Is this data of any use to anyone? Who do I send it to?
I'd be glad to run data logging reports on the USB port as this thing communicates in Windows, although the MS logger function does truncate some of the traffic...