MagTek PORT POWERED SWIPE READER - Manual (Page 27)

Section 4. Properties and Commands
21
As with the switch settings, the sentinel parameters can be discovered by sending the
corresponding command for that value immediately followed by a CR. For instance, in order to
determine the present setting of the 7-bit track 3 start sentinel, send the following command:
<ESC>S5<CR>
The Reader will respond with the value, in nibbles:
<ACK><ESC>S526<CR>
Any ASCII character from 0x00 to 0x7F can be used as a sentinel. To change the value of the 7-
bit track 3 start sentinel to “!” (0x21), send the following command:
<ESC>S521<CR>
Again, as with the switch commands described above, send the Upload (UP) command followed
by the Reset (RS) command to complete the transaction.