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For all ZyXEL modems:  Type "ATI" in a terminal program.  If the response is 
"1496", then type "ATI1" in the terminal program.  (Alternatively, 
U-1496/U-1496S or U-1496+/U-1496S+ models can display the ROM revision via 
the product information screen.)

Paul Platt (pep@cypress.com) writes:
There is an easier way to get the firmware revision if you have an S or
S+ model ZyXEL. Select the diagnostic display screen. Move all the way
right. The firmware revision will show on the display.  Note that you
cannot always trust the corner arrow markers. They sometimes indicate
that there is no "next screen" when a "next screen" actually exists.

At some point, the response should have something like "U1496E V 6.13 M".

The "V 6.13" indicates the ROM revision number (i.e. Version 6.13)

The "M" character at the end indicates   1Mbit ROMs.
The "K" character at the end indicates 512kbit ROMs.
The "P" character at the end indicates that the modem is a "Plus" modem.