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

11.8

General issues in communication

Reduce the rate of communication request per second as far as possible. Utilize the
maximum user data length for communication requests, for example by grouping several
tags or data areas in one read request.
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:
http://www4.ad.siemens.de/view/cs/de/1651770, article ID 1651770
Your calls of communication requests should allow the event-driven transfer of data. Check
the data transfer event only until the request 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 using a conditional jump if you do not 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 in 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 PLCs, or external 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 corresponding SFB 13
"BRCV" at the communication partner.
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 corresponding SFB 9
"URCV" at the communication partner.
S7-400H
System Manual, 09/2007, A5E00267695-03
11.8 General issues in communication
Communication
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