Estimating Transmission Time; Permissible Cable Lengths; Approximate Message/Bus Cycle Calculation With Cyclic Data Exchange - Balluff BTL5-T110 Series Technical Description

BTL5-T110...
Configuration and Profibus Connection –
Transducer Class 3/4
3

Estimating transmission time

Since there are dependencies
between the Profibus cable length
and the baud rate, it is important
to estimate the cycle time.
3.1

Permissible cable lengths

Max. baud rate Cable
in kBit/s length BKS-S103...
9.6
19.2
45.45
93.75
187.5
500
1500
3000
6000
12000
Table 3-1: Max. lengths for RS-485
Baud rate Max.
in kBit/s capacitance
9.6 15 nF
19.2 15 nF
45.45 3 nF
93.75 3 nF
187.5 1 nF
500 0.6 nF
1500 0.2 nF
3000 -
not allowed
12000
Table 3-2: Max. stub line lengths for
RS-485
These numbers are for the total
number of stub lines, i.e. for a
cable with 187.5 kBit/s and 32
connected devices the stub line
can be an average of approx. 1
meter in length for each device. At
bits rates over 1.5 MBit/s stub
lines are not permitted.
6
english
3.2 Approximate message/bus
cycle calculation with cyclic
Data exchange
3.2.1 System response time
The system response time of a
Profibus system depends
essentially on the following
factors:
1200 m
- T (the response time after
1200 m
SDR
which a station can reply)
1200 m
- The selected transmission speed
1200 m
- Min_Slave_Interval
1000 m
- The agreed upon net data length
400 m
200 m The weakest link on the bus
system "brakes" the entire bus,
100 m
since there is only one
100 m
transmission rate on the bus
specified by the master. On the
100 m
other hand the Min-Slave_Interval
allows you to implement an
equidistant time response, since
the Class 1 master waits at least
Max. length of
for the time of the longest
stub lines
Min_Slave_Interval after each bus
500 m
cycle until it begins a new bus
500 m
cycle.
100 m
This sample calculation contains
100 m
no possible diagnostic telegrams
33 m
and telegram repetitions. The pro-
20 m
gram run times in the master are
6,6 m
not relevant here, since the master
ASICs generally work completely
autonomously and therefore only
the run times between the PROFI-
BUS ASICs (master and slave
Fig. 3-1: Overview of relevant bus times
ASIC) are considered. The program
run times are however important
for the system response time.
1 TBit at 12,000 kBit/s =
1/12,000,000 bits/s = 83 ns
1 character consists of 11 bits
(1 start bit, 1 stop bit, 1 parity bit,
8 data bits)
=> 1 character corresponds to
83 ns * 11 = 0.913 µs
In data exchange the telegram
header consists of 9 bytes. The
bus rest times are assumed to be
1
T = 33 TBit and T
syn
should at 1500 kBit/s be set at
36 TBit). The propagation times of
the signals on the bus can be
ignored. In addition, a typical
value of 30 TBit is assumed as
T for a baud rate of greater than
SDR
1500 kBit/s. The T
depending on which ASIC is used.
The overhead of a message cycle
(T ) according to Fig. 3-1:
MC
Overview of relevant bus times,
consists of the total of the bus
times plus the telegram header.
1 The Min_Slave_Interval is the time
between two polling cycles in which a
slave can exchange data with the master.
= 75 TBit (T
id1 id1
time varies
SDR