EFJohnson 3000 SERIES - Manual (Page 119)

TELEPHONE INTERFACE MODULE (TIM)
14-9
oscillator of U211 running at
≈
3.7 kHz. With U244,
pin 12 low the output of U210, pin 9 is a “1” (U210,
pin 12 is constantly being retriggered). If a transient
“0” during a logic “1” is encountered at the E-Lead,
U210, pin 7 is retriggered causing U211, pin 3 to go
high for 5 ms. This will not change the output of
U211, pin 9 because it is being retriggered. If U211,
pin 3 stays high longer than 10 ms, U211, pin 9 goes
low. For this to occur, the transient would need to
exceed 10 ms.
Figure 14-13 4-WIRE E&M BLOCK DIAGRAM (-680)