Siemens S7-200 - Principle of Operation; Acquisition and Sending of Process Values with Time Stamp

Principle of Operation
Micro Automation Set 14 Entry ID 21062588
V2.0 05/18/05 13/35
Copyright © Siemens AG 2005 All rights reserved
Set14_DocTech_V2.0_en.doc
4 Principle of Operation
4.1 Acquisition and sending of process values with time stamp
Table 4-1
No. Function Comment
1.
The central processing unit of S7-200
“Basis” acquires and stores process
values with time stamp to the second in a
reserved memory area of the S7-200
CPU.
2.
The generated data fields are written into
a data block of the S7-200 CPU at
intervals of one second. A data field
comprises two bytes for the measured
process value and six bytes for the time
stamp.
3.
The data block with the stored data fields
acts as send buffer. It buffers 20 data
fields before the send job is started. To
ensure consistency of the data at the
receiver, the actual send procedure is
divided into two steps:
• Step1: Sending all 20 data fields +
consistency byte (status 0) to the
receiver
• Step2: Sending the consistency byte
(status 1) after successful send
procedure of step 1. It is only now
that consistent availability of the data
at the receiver is ensured.
S7-200 “Basis”
78 98
55
DB1
55
78
98
S7-200 “Basis”
hour
minute
second
day
month
year
value
Data field
Step 1
Connection1:
192.168.2.XXX
Transfer data:
20 data records
Consistency byte (0)
Step 2
Connection1:
192.168.2.XXX
Transfer data:
Consistency byte (1)