How to extract n frames from a GROMACS trajectory

Question

[simple_dozer]

I want to extract n frames in pdb format from say, the last half of a gromacs trajectory. How do I do this? This is when I have 50000 as my nsteps in the mdp file I used for running grompp, before I did energy minimisation using mdrun. I tried playing around with -b and-e arguments for gmx trjconv but I get and error saying:

...
trn version: GMX_trn_file (single precision)
Last frame         -1 time    0.000   
WARNING no output, last frame read at t=474
...

For example, I may get this error by running gmx trjconv -f the-em.trr -o the-em.pdb -b 0 -e 400 -dt 10