BTL5-T110...
Configuration and Profibus Connection –
Transducer Class 3/4
7
english
3
Estimating transmission time (cont.)
4
Transmitting data in Data_Exchange
From this follows:
T
MC
= (2 * length of header in bytes) * 11 bits + T
SDR
+ T
SYN
+ T
ID1
T
MC
= (198 bits + 30 bits + 33 bits + 75 bits) TBit
= 336 bits * 83 ns = 28 µs
The overhead caused by the telegram header and bus rest times is therefore 28 µs.
The approximate time for a message for Example 1:
T
MC
= 28 µs + number of net data (here 1 * po 1 * velocity = 4 bytes + 4 bytes)
T
MC
= 28 µ + 8 µs = 36 µs/slave
Approximate time for a message for Example 2:
T
MC
= 28 µs + number of net data (here 4 * po 4 * velocity = 16 bytes + 16 bytes)
T
MC
= 28 µ + 32 µs = 60 µs/slave
T
MC
= Time of a message cycle
(Request te T
SDR
+ reply from slave)
T
BC
= time of a bus cycle; calculated by adding the message cycles. In multi-master mode the individual cycles
are added, and for each master a FDL_Request telegram (for detection of a new station) and a token telegram are
considered. An additional time for calculation applies for
1. the token telegram (3 char T
id1
[75 TBit]) an approximate time of 3 µs + 6 µs = 9 µs
2. the FDL_Reques_Status (6 char Tid1 [75 TBit]) an approximate time of 6 µs + 6 µs = 12 µs
The approximate bus cycle for Example 1.
T
BC
= (36 µs * 1 Slave) + 12 µs + 9 µs = 57 µs
The system response time is of course significantly longer.
For the sake of simplicity the following may be assumed:
To send a character at 12,000 kBit/s, an overhead of 28 µs + ca. 1 µs per net datum is needed. An exact (though
somewhat more complex) calculation of the transmission times is found in IEC 61158.
The sensor in- and output bytes are sent according to the following telegram scheme.
Standard Telegram 81:
1 position value selected from the GSD
Word number
1
2
Input value
STW2
G1_STW1
Word number
1
2
3
4
5
6
Output value
ZSW2
G1_ZSW1
G1_XIST1
G1_XIST2
Word number
1
2
Input value
STW2
G1_STW1
Word number
1
2
3
4
5
6
7
8
Output value
ZSW2
G1_ZSW1
G1_XIST1
G1_XIST2
Position 1
