PCL
Pleiger Elektronik GmbH & Co. KG
www.pleigerelektronik.de
Fon +49 2324 398 333
6
/
65
Pleiger Control Loop
Manual version 2018-03
3
The Data Bus System
The PCL data bus is a master slave system using a communication protocol similar to the Modbus
protocol but providing special function types and a very tight timing for increased performance. Hereby
a fast data transfer is achieved even at low baud rates. The communication nodes connected to the
data loop are called nodes.
3.1 PCL Master Function
In most applications two independent master controllers (A and B) handle the communication with the
nodes at both ends of the data bus. The polling sequence is the major difference between both bus-
masters. While A is starting with the first node of a data loop counting upwards, B starts with the last
one and then counts downwards.
Note: The sequence used for polling the nodes is parameterised by the user. This sequence must be
identical to the physical sequence of the slave units (EHS) on the bus. Otherwise a proper fault
localisation is not guaranteed.
Both masters are connected via CAN bus. This connection is used to copy any information that one of
the masters receives to the other the other
master immediately. Therefore both masters’ data are al-
ways identical.
3.2 Multiple Data Busses
To increase the communication speed and to improve the availability in case of multiple errors each
busmaster can serve up to 8 data loops simultaneously. All data loops have identical functions. The
number of data loop and the nodes connected to each data loop can be selected by several parame-
ters. Each data loop can serve up to 100 nodes and up to 250 nodes may be connected to a busmas-
ter resp. a pair of busmasters. If more than 60 nodes shall be connected to a busmaster at least two
data loops are recommended. Four to eight data loops are recommended for 250 nodes.
3.3 Function PCL Slave
The PCL slave interface is the most important link in the PCL safety chain. It is not only acting as a
communication slave but also takes over the following tasks:
•
Separating the data bus loop sections electrically
•
Acting as a repeater
•
Selecting the valid busmaster
•
Control of the actuator resp. operating
3.3.1 The physical layer
Electrically the slaves perform a point-to-point communication on two RS485 ports A and B. Both
ports are optically isolated, overload protected and connected by a simple logic. Special 485 drivers
guarantee a differential output signal with extraordinary high amplitude and an improved signal-to-
noise ratio. Each node acts as a signal repeater.
Because of the additional protection devices even a short circuit between the 230V AC supply and the
data bus will not cause a permanent damage of the slave but of course an interruption of the data link.
For data transmission a shielded twisted pair cable with at least 0,75 mm² cross-section is required.
This twisted pair is part of the PCL bus cable. Other cable types (5x2,5) are going to be tested.
3.3.2 Data communication and address selection
The protocol is quite similar to the Modbus protocol. Only the function types are different and the tim-
ing is faster to achieve optimal performance. The most frequently used message exchange is based
on the master’s status request (4 bytes) and the slave’s status response (6 bytes). Using a baud rate
