MagTek P-SERIES - Manual (Page 17)

Section 2. Commands, Formats, Timing
11
Track 2 data may be represented as follows:
Where
SS = Start Sentinel; "%" for Track 1; ";" for Track 2
Data = Track Data in track order that is, Track 1 then Track 2
ES = End Sentinel; "?"
Sensor = "0" no card in reader
"1" card present in reader (rear sensor blocked)
If there is an error in one of the tracks, the "Track Data" field will be replaced with "E" (0x45)
An example of a card insertion or removal is as follows:
Where
1 = Sensor blocked
0 = Sensor unblocked
The following is an example of a good read on withdrawal of a card:
Where 0 represents the sensor unblocked.
The following is an example of a bad read on Track 1 and a good read on Track 2 on withdrawal
of a card:
SS Track 2 Data ES Card Sensor
STX ESC 1 CR ETX
STX ESC % E ? ; Track 2 Data ? 0 CR ETX
Optional
STX ESC % Track 1 Data ? ; Track 2 Data ? 0 CR ETX
Optional