ULINK2 根据PID分为两个可用固件版本：2720 或者 2722

lsusb 可以看到

Bus 001 Device 011: ID c251:2720 Keil Software, Inc. 4-Port USB 2.0 Hub

2720
ULINK2 固件版本为 1.42 或者更低

ULINK2 固件版本为 2.00 或者更高，但是 用UL2_Configure工具配置成了 ulink boot模式

同 MDK-ARM V4.60 之前的版本兼容

2722
ULINK2 固件版本为 2.00 或者更高, 配置成了CMSIS-DAP兼容模式， 默认的boot模式也是 CMSIS-DAP

2727
没有固件，只有bootloader

CMSIS-DAP is the interface firmware for a Debug Unit that connects the Debug Port to USB. Debuggers, which execute on a host computer, connect via USB to the Debug Unit and to the Device that runs the application software. The Debug Unit connects via JTAG or SW to the target Device. ARM Cortex processors provide the CoreSight Debug and Trace Unit. CMSIS-DAP supports target devices that contain one or more Cortex processors.

CMSIS-DAP can be used with:

MDK-ARM V4.54. (or later).
The Debug Unit in use must support the Firmware for the CoreSight Debug Access Port (CMSIS-DAP). The firmware gets implemented by the vendor of the device or debug adapter.
CMSIS-DAP firmware needs to be version V1.10 or newer to support SWO trace. Verify the firmware in CMSIS-DAP JTAG/SW Adapter.

ULINK2 and ULINK-ME firmware updates have always been backward compatible to all previous µVision versions but unfortunately firmware version 2.01 is an exception. The firmware version 2.01 and later is now CMSIS-DAP compatible by default.