Rockwell Automation 1771-DCM USER MANUAL 1771-DCM - Manual (Page 36)

Calculating Transfer Time
Chapter 7
7Ć2
Total Scan (PLC-2) = Program + Processor I/O + Remote I/O
Total Scan (PLC-3) = Program + Remote I/O
1771ĆDCM
The delay from the time the 1771-DCM receives data until it is ready for
data transfer is 15 ms. During this time, it detects errors and changes in
status and formats data.
Local Processor
The local processor reads data from or writes data to the 1771-DCM using
block transfer instructions, regardless of how data is transferred between
1771-DCM and supervisory processor. The time required is equal to the
program scan plus block transfer time which depends on the number of
words transferred.
Time for PLCĆ2/05 PLCĆ2/15 PLCĆ2/30 (Local)
Program Scan 15 ms/K 18 ms/K 5 ms/K
I/O Scan 1 ms 4 ms 0.7 ms
Block Transfer 0.08 ms/W 0.08 ms/W 0.08 ms/W
W = Number of Words Transferred
Total Scan = Program and I/O Scan + (Multiplier) x (Number of Words
Transferred)
See the appropriate processor manual for times for other processors.
Addition of Block Transfer Modules
Block transfer modules in the local I/O chassis can lengthen the I/O scan
time by the time required for each to perform a block transfer (worst
case). The delay depends on the number of block transfer modules
waiting to transfer and the number of words each will transfer. Calculate
the additional block transfer time for your processor as follows for each
block transfer module:
Time (Local) = Multiplier x (Number of Words Transferred)