Rockwell Automation 1775-S5_SR5,D17756.5.5 User Manual PLC-3 FAMILY I/0 - Manual (Page 73)

DH and DH+ Communication
Chapter 4
4-13
Calculating DH and DH+ Communication Times
The time required for the scanner in a local station to send or receive a
message with a scanner in another station on a DH or DH+ link generally
depends on the number of:
stations on the DH+ link
messages transmitted from active stations
bytes of data of all transmitted messages
message requests that are queued ahead of the subject message
Table 4.C lists typical I/O scan times and throughput times for a DH or a
DH+ configuration. The throughput time is the execution time for the
DH or DH+ message. This time starts with setting the enable bit and ends
with setting the done bit in the ladder program of the station sending or
receiving the message.
When calculating these times, the DH/DH+ link had one other station on
the link, and the message instruction was sending 10 words to the second
station. The second station was not transferring any words.
Table 4.C
Throughput Times for a DH or DH+ Configuration in Milliseconds
Number of Active
I/O Ch l
Throughput Times
I/O Channels
DH DH+
0 109 66
1 110 67
2 113 67
3 119 67
Important: I/O communication channels can affect the time required to
complete DH+ functions initiated on the same scanner. If any of the
devices in the I/O rack lists are not responding to the scanner (retrys), then
the time to execute some programming terminal DH+ functions, such as
ladder logic searches, will take longer.