MagTek PORT POWERED SWIPE READER - Manual (Page 27)

Section 4. Properties and Commands
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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:
S5
The Reader will respond with the value, in nibbles:
S526
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:
S521
Again, as with the switch commands described above, send the Upload (UP) command followed
by the Reset (RS) command to complete the transaction.