wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

46-6 Industrial Communication Systems
Safety PLC
Standard PLC
FIGURE 46.8
Split assignment of safety and standard tasks.
standard tasks may also be assigned to different controllers and networks as shown in Figure 46.8.
Any so-called acyclic communications between devices and controllers or supervisors such as programming
devices are intended for configuration, parameterization, diagnosis, and maintenance purposes.
46.2.4 Cyclic Communication PDU
DP-DP coupler
The PDU (protocol data unit) structure of the Profibus I/O data exchange message is exactly the same
whether or not there is any safety data in the message. This structure has separate data areas assigned
to each module in a modular slave device, facilitating the intermixing of safety and standard modules
as shown in Figure 46.9, where the safety modules map onto the Profisafe data areas in the PDU. Data
associated with a safety module are contained within an area of the PDU assigned only to that safety
module and affects no other data areas.
As has been illustrated here, standard and safety devices may be intermixed on the same network and
standard and safety modules may be intermixed within the same modular slave device. The only change
required within the standard I/O data exchange PDU structure was to include error-detection information
within the data area associated with a safety module. No changes were made to the data areas of
standard modules. This isolated area of change allowed the same implementations of Profisafe across
Profibus-DP, Profibus-PA, and Profinet (Figure 46.10).
The detailed structure of the safety data and how it affects the desired error-detection solutions will
be described in the following sections. The data structure supports the checks required to detect all the
error types listed in Figure 46.5. Figure 46.11 shows the general format of the safety data while Figures
46.12 and 46.13, respectively, show the details of the control byte sent in the safety data from the controller
and the status byte in the safety data returned from the device/module.
The mechanisms used for error detection, i.e., consecutive numbering, timeout with receipt, code
name for sender/receiver, and data consistency check (cyclic redundancy check, CRC), will be briefly
discussed in the following sections.
Head
module
Safety
module
Normal
module
Normal
module
Safety
module
Safety
module
Normal
module
Safety
module
Data
Safety
data
Normal
data
Normal
data
Safety
data
Safety
data
Normal
data
Safety
data
FIGURE 46.9
Intermixed standard and safety I/O modules.
© 2011 by Taylor and Francis Group, LLC