Allen-Bradley A-B QUALITY DL20 Series User Manual page 87

D0
D1
D2
D3
D4
D5
D6
D7
D8
D9
MS0
MSI
t0 t1
t0: The value on the data lines is zero, the DL20 ignores this
value because there is no message zero.
t1: The data lines change state, from zero to seven.
t2: The message trigger seven is queued up.
t3: The data lines change state, from seven to twelve.
t4: The message trigger twelve is queued up.
t5: The data lines change state, from twelve to zero.
You can tie the strobe lines high because message zero is ignored. Keep all
data lines at zero, until you want to trigger a message. The last message is
only triggered once --no matter how long the data lines stay in their state
(DL20 rejects unchanging data). To trigger the same message twice in a row,
the message must be triggered, the data lines must return to a zero state, the
message can then be retriggered. The following diagram shows how
message 7 could be triggered once and message 12 triggered twice using the
connections shown in Figure 7.4.
t2 t3 t4
Chapter 7
The Parallel Port
t5 t6 t7
t6: A zero is presented to the DL20. Twelve cannot be presented
here, because the DL20 will not queue the same message twice
in a row, as long as MS0 and MS1 remains ON. In this example,
MS0 and MS1 are always ON.
t7: The data lines change state, from zero to twelve.
t8: The message trigger twelve is queued up.
t9: The last message has been triggered, the data lines may remain
in their previous state or they may change back to zero.
t8 t9
7–15