Prevent Vim setting Application Keypad Mode on my VT520

Question

I am attempting to use Vim via a serial console and every time I launch it, vim sends ESC= and instructs the console to enable Application Keypad Mode and my numpad stops working.

How do I stop it doing this? I've seen multiple answers that say "just turn it off in the settings" but Vim keeps turning it back on again!

Answer 1

Try unsetting 't_ks' and 't_ke' in your vimrc:

" Prevent Vim from setting application keypad mode.
set t_ks= t_ke=

If you'd like, you can do that conditionally, based on the TERM=vt520 setting:

" Prevent Vim from setting application keypad mode on a VT520.
if &term ==# 'vt520'
  set t_ks= t_ke=
endif

These sequences are used by Vim to start and end "keypad transmit" mode.

Vim documentation also mentions those in this help section and, in particular, recommend disabling them (setting them to an empty string) in cases where they cause trouble:

Some termcap entries are wrong in the sense that after sending 't_ks' the cursor keys send codes different from the codes defined in the termcap. To avoid this you can set 't_ks' (and 't_ke') to empty strings. This must be done during initialization, otherwise it's too late.