If you can read a PDF. file check this link. http://www.dainst.com/info/circuits/msd6a_02.pdf
This site has a good schematic of the MSD 6A. I have copied and pasted some of the text information from this site.
3.5 Tachometer Output and Connector (Internal Yellow Wire)
Note that if a tachometer was connected previously to the stock coil, it will probably not function correctly
with the MSD-6A. The tachometer is expecting one pulse for each combustion cycle, whereas the MSD
will deliver multiple pulses. Because of this, MSD units have a separate tachometer output. On the
MSD-6A, it is a connector (the internal wire is yellow). This output is driven by an open collector NPN
transistor with a 270 ohm, 1 watt pull-up resistor to the supply voltage (12V). This output could easily be
used as an input to an EFI computer if a level translation to +5V is performed.
4.5 Tachometer Output
The duty cycle of the tachometer output was measured as a function of input frequency. The input signal
was a TTL level squarewave applied to the violet and green wires (magnetic pickup input). The data is
presented in Table 4.
The time delay between the trigger input and the tachometer output was measured to be 6 uS.
B. Zublin MSD6A_02.DOC rev 0.2 06MAY97 11
Table 4 Tachometer Output Duty Cycle vs. Input Frequency
Measured Data - Model 6200
Input Frequency (Hz) Tachometer Output
(Duty Cycle (%) 1
1 The duty cycle (%) is defined as the time that the signal is at +12V relative to the period of the signal.