ABB AC 800M

RS485 network limits Section 2 Transmission Technology
28 3BDS009029R5001 B
Calculation of a bus length against of two transmission rates:
A transmission rate of 3 Mbit/s shall be used. The transmission rates in the table
close to this value are 1.5 Mbit/s with l
1,5Mbit/s
and 12 Mbit/s with l
12Mbit/s
. Thus,
you can calculate the line length l
3Mbit/s
:
RS485 network limits
Up to 32 nodes (master and slaves) can be grouped in a single segment. When using
more than 32 nodes, several segments linked to each other through repeaters (power
amplifiers) are needed. Note that the repeater has to be considered as another bus
node, since the integrated bus drivers are an additional load on the bus. It is
recommended to use not more than three series-connected repeaters on a line.
However, the number of repeaters in a single PROFIBUS network is limited to 9.
The maximum admissible distance between two bus stations in each PROFIBUS
network can be calculated as follows:
(Number of repeater + 1) * Segment length
Number of repeater = The maximum number of repeaters connected in series.
Example: The repeater manufacturer’s specifications allow nine repeaters to be
connected in series.
The maximum distance between two bus stations at a data transfer rate of 1500
kbit/s is then as follows:
=> (9 + 1) * 200 m = 2000 m
l
3Mbit/s
: l
1.5Mbit/s
+ 3Mbit/s - 1.5Mbit/s()
l
1.5Mbit/s
- l
12Mbit/s
1.5Mbit/s - 12Mbit/s
-------------------------------------------------
×
l
3Mbit/s
185m≈
=