MagTek TTL - Manual (Page 24)

I/O Interface for TTL Magnetic Stripe Readers
20
It is necessary to write a program that will:
1) go through memory, starting from the end that has the start sentinel, and
2) decode the characters (which may be forward or backwards depending on which way the card
was swiped and not on a byte boundary), and
3) check for errors then transmit the characters forward back To the host.
Another problem is that the LRC will assume the same value as the Start Sentinel some of the
time, and when it does, then the character adjacent to it has to be tested to see if it is an end
sentinel. If it is an end sentinel, then this character is an LRC otherwise it is a Start Sentinel. In
this way you can make sure which end is really the beginning. See Appendix D, Bidirectional
Reading.
CARD READBACK AMPLITUDE VARIATIONS
Causes of readback amplitude variations are as follows:
•
Speed of the card - the faster, the more amplitude
•
Head contact, head pressure, smoothness of stripe, bumps
•
Quality of oxide
•
Orientation of oxide (polarization)
•
Thickness of oxide
•
Type of oxide, low or high coercivity
To check the amplitude variations, connect a scope between ground (pin 5) and drive or gain set
(pin 9 or 11), on the F2F read/decode IC. Swipe an encoded card, and check the scope for the
variations above.
The waveshape should be similar to Figure 2-1. For pin locations refer to Appendix B,
Connection diagram (Pin Outs).