General Issues Regarding Communication - Siemens SIMATIC S7-400H System Manual

Communication

14.8 General issues regarding communication

14.8 General issues regarding communication
Reduce the rate of communication jobs per second as far as possible. Utilize the maximum
user data length for communication jobs, for example by grouping several tags or data areas
in one read job.
Each request requires a certain processing time, and its status should therefore not be
checked before this process is completed.
You can download a tool for the assessment of processing times free of charge from the
Internet at:
Service & Support (http://www.siemens.com/automation/service&support) ID 1651770
Your calls of communication jobs should allow the event-driven transfer of data. Check the
data transfer event only until the job is completed.
Call the communication blocks sequentially and stepped down within the cycle in order to
obtain a balanced distribution of communication load.
You can by-pass the block call command by means of a conditional jump if you do not want
to transfer any user data.
A significant increase in communication performance between S7 components is achieved
by using S7 communication functions, rather than S5-compatible communication functions.
As S5-compatible communication functions (FB "AG_SEND", FB "AG_RECV", AP_RED)
generate a significantly higher communication load, you should only deploy these for the
communication of S7 components with non-S7 components.
AP_RED software package
When using the "AP_RED" software package, limit the user data length to 240 bytes. If
larger data volumes are necessary, transfer those by means of sequential block calls.
The "AP_RED" software package uses the mechanisms of FB "AG_SEND" and FB
"AG_RCV". Use AP_RED only to link SIMATIC S5/S5-H controllers or third-party devices
which only support S5-compatible communication.
S7 communication (SFB 12 "BSEND" and SFB 13 "BRCV")
Do not call SFB 12 "BSEND" in the user program more often than the associated SFB 13
"BRCV" at the communication peer.
S7 communication (SFB 8 "USEND" and SFB 9 "URCV")
SFB 8 "USEND" should always be event-driven, because this block may generate a high
communication load.
Do not call SFB 8 "USEND" in the user program more often than the associated SFB 9
"URCV" at the communication peer.
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System Manual, 03/2012, A5E00267695-11
S7-400H