Rockwell Automation 2706 DL20 SERIES G USER MANUAL - Manual (Page 84)

Chapter 7
The Parallel Port
7–12
Variable data is transferred up to eight bits at a time. The high order data bits
are strobed over, then the low order data bits are strobed over. Only then is
the data queued up. The strobing order is important; high order data bits
must be strobed in first. The timing of a typical data transfer using binary
data could look like this:
t7 t8 t1 t2 t3 t0 t6 t5 t4
D0
D1
D2
D3
D4
D5
D6
D7
D8
D9
MS0
MSI
t4: The data lines can now change to indicate the low order byte.
The DL20 will ignore the data lines because both MS0 and MS1
are OFF. t4 can last for any length of time.
t5: Strobe line MS0 is turned ON.
t6: The data lines are latched in and they are interpreted as low
order data because MS1 is OFF and MS0 is ON. The data read
in is equal to the sum of the values of the bits. We can look at
the table 7.B and see that the value strobed over is 64
+ 16 or 80. The DL20 will now queue up a variable since both
high order data and low order data have now been strobed in.
The value of the variable queued will be 10,752 + 80 or 10, 832.
t7: Strobe line MS0 is turned OFF.
t8: As long as MS0 and MS1 are OFF, the DL20 ignores the data
lines.
t0: The binary code for 10,832 is 0010 1010 0101 0000. During t0
the first eight bits of the data word are set up on the parallel
input lines: D5, D3, and D1 are ON and D7, D6, D4, D2 and D0
are OFF. Since both strobe lines are OFF, the DL20 ignores
the data lines.
t1: Strobe line MS1 is turned ON, strobe line MS0 is left OFF. This
tells the DL20 that we are entering high order data.
t2: The data lines are latched in and they are interpreted as high
order data during the period t2 because MS1 is ON and MS0 is
OFF. The data read in is equal to the sum of the values of the
bits. We can look at the table 7.E and see that the
value strobed over is 8,192 + 2,048 + 512 or 10,752. t2 should
be long enough so data can be sampled by the DL20. As noted
before, this sampling time is user-selected. If edge triggered
sampling is used, the edge should occur during t2.
t3: Strobe line MS1 is turned OFF.
Transferring Variable Data