Here's some code that I based on the links below with changes for the values you gave for F0 and F1. (It sounds like yours is wired in a better fashion than theirs.) It's a subroutine for calculating the command to send.

#!/bin/env perl
use strict;
use warnings;

use List::Util qw(sum0);
use List::MoreUtils qw(uniq);

{
  my %buttons = (
    '0' => 16,
    '1' => 8,
    '2' => 4,
    '3' => 2,
    '4' => 1,
  );

  sub buttonsPressed {
    my ($cab, @pressed) = @_;

    my @values = map { 0 + $buttons{$_} } uniq @pressed;
    return sprintf "<f %d %d>", $cab, 128 + sum0 @values; # Yes, these
+ two lines could be combined
  }

}

my $cab = 1108;

my @tests = (
  [ 0 ],
  [ 1 ],
  [ 2 ],
  [ 0, 1 ],
  [ 0, 2 ],
  [ 1, 2 ],
  [ 0, 1, 2 ],
);

for my $test ( @tests ) {
  print "@{$test} : ", buttonsPressed( $cab, @{$test} ), "\n";
}
__END__
0 : <f 1108 144>
1 : <f 1108 136>
2 : <f 1108 132>
0 1 : <f 1108 152>
0 2 : <f 1108 148>
1 2 : <f 1108 140>
0 1 2 : <f 1108 156>
[download]

Related links?

• http://www.trainboard.com/highball/index.php?threads/introducing-dcc-a-complete-open-source-dcc-station-and-interface.84800/page-4
• https://github.com/DccPlusPlus/BaseStation/blob/master/DCCpp_Uno/SerialCommand.cpp