Black Channel constraints
Even though PROFIsafe uses the "Black Channel" principle, there are some basic features of PROFIBUS and PROFINET that were considered for its design.
One such feature is the cyclic communication between a bus controller and its associated field devices (send and receive response principle). This polling operation will immediately detect any failed device. PROFIsafe adopted this principle and thus can omit a separate "sign-of-life" mechanism.
The other feature is the 1:1 communication relationship between a bus controller and its associated field devices. PROFIsafe also adopts this principle to ensure the authenticity of messages. However, this restricts an F-Module (Subslot) within a modular field device to be accessed by only one F-Host.
A "Black Channel" may comprise several types of transparent network components such as switches, routers, links, and wireless transmission channels. For PROFIsafe, minor constraints exist in order to meet the SIL3 requirements.
Any kind of switch is permitted but only 100 may be connected in a row. The F-Address space within a PROFIsafe island must be unique. Connected islands with the same F-Address space must be separated by multi-port routers. There are no known restrictions for links such as from PROFINET to PROFIBUS and from there to the intrinsic safety version MBP-IS (Figure 6).
Wireless & Security
Wireless transmission is permitted as long as sufficient availability (no nuisance trips) and security can be guaranteed.
PROFIsafe specifies certain security requirements for wireless transmission and for wired networks that are connected to industrial Ethernet backbones or the Internet (open networks).
Fieldbus communication in general uses many different data types for information transfer (see literature box). In order to reduce complexity, PROFIsafe offers a reasonable subset.