Product manual

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Input Output
Physical I/O
Ladder
Program
PLC I/O Mapping
Slave 1
Slave 2
Slave 3
PLC
Physical
Actuator 1
Figure 2-1a: Plant wiring
conventional comms. systems
I/O scanning
Physical
Actuator 2
Physical
Actuator 3
Physical
Actuator 4
I/O
Mod-
ules
Input
Input
Output
Output
Slave 4
Figure 2 PROFIBUS compared with convetional comms. systems
PROFIBUS-DP distinguishes between master devices and slave devices. It allows slave devices
to be connected on a single bus thus eliminating considerable plant wiring typical with
conventional communications systems. The Figure above compares the two systems.
Master devices determine the data communication on the bus. A master can send messages
without an external request when it holds the bus access rights (the token). Masters are also
called active stations in the PROFIBUS protocol.
Slave devices are peripheral devices. Typical slave devices include input/output devices, valves,
motor drives and measuring transmitters. The 2408
f and 2404 f series Temperature Controllers
are intelligent slaves. This means they will only respond to a master when requested to do so.
PROFIBUS-DP is based around the idea of a ‘cyclical scan’ of devices on the network, during
which ‘input’ and ‘output’ data for each device is exchanged.
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The process of reading the inputs and writing to the outputs is known as an I/O data exchange.
Typically, the parameters from each slave device will be mapped to an area of PLC input and
output registers, or a single function block, so that the controlling ladder logic or program
interfaces with the device as if it were an internally fitted module. It is NOT necessary,
therefore, for the programmer to know anything about the physical network. The process of
network configuration is usually performed using a PC based program which allows the devices
on the network to be defined and device parameters to be mapped into the PLC registers or
function blocks.
The cyclical scan occurs in the following order:
1. Values from each slave device, ‘Input Data’, are first scanned over the network into a pre-
defined set of input registers in the master controller. Such values might be a set of digital
input readings for a digital input unit, or the measured temperature and alarm status from a
PID controller.
2. The master then runs its control program, (such as a ladder logic program) using the input
data read from the slave devices.
3. The master writes output values (output data) into a pre-defined set of output registers. For
example, one of the digital inputs read in the input data might be used to select one of a set of
setpoints to be sent to the PID controller.
4. These outputs are then written to each slave device, and the scan-process-write cycle repeats.
Typically no more than 32 bytes of input data and 32 bytes of output data are exchanged for each
device during the data exchange. Some PLC masters allow no more than this, although the