Profibus:FDL
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[edit] UART octet
Profibus is based on UART (Universal Asynchronous Receiver/Transmitter) communication. An element of each telegram (frame) is a UART character defined by standard ISO 11777 and ISO 2022 for asynchronous communication. A UART character is a sequence of 11-bits starting with a Start bit (low level), 8~data bits (the first bit is LSB - Least Significant Bit), an even parity bit and a Stop bit (high level).
[edit] Telegrams
- Telegrams are used to carry user data (e.g. values of sensors and actuators), service commands, configuration and diagnostic information as well as token passing between masters in a logical ring.
- Profibus defines 5 types of frames. Every frame begins with a Start Delimiter and terminates with an End Delimiter, except the Token and Short Acknowledgement (SC) frames.
- Hamming distances between Start Delimiters are 4. It means that Start Delimiters could be confused only if at least 4 bits were transmitted with errors.
- Furthermore, a frame contains Source (SA) and Destination Addresses (DA) (except the SC telegram). Frames with variable length have two bytes carrying redundant information about the frame Length (LE). The Frame Control (FC) field is use to indicate the frame type (whether request or response), station type or is used as a mechanism for preventing loss or multiplication of messages. Frames containing End Delimiters also have a checksum field Frame Check Sequence (FCS).
[edit] Frame Control
The frame control octet indicates:
- Type of transmitting message (request, acknowledgement or response frame)
- Information preventing loss and multiplication of messages
- Station type and FDL state
b7 b6 b5 b4 b3 b2 b1 b0
+-----+-----+-----+-----+-----+-----+-----+-----+
! ! 1 ! FCB ! FCV ! !
! Res !Frame!-----+-----+-- Function --!
! ! 0 ! Stn-Type ! !
+-----+-----+-----+-----+-----+-----+-----+-----+
Res: Reserved (the sender shall be set binary "0", the receiver does not have to interpret)
Frame Type:
1 Request, Send/Request Frame
0 Acknowledgement, Response Frame
b6 = 1:
FCB Frame Count Bit: 0/1, alternating
FCV Frame Count Bit valid:
0 alternating Function of FCB is invalid
1 alternating Function of FCB is valid
b6 = 0:
Stn-Type: Station Type and FDL Status
b5 b4 <-- Bit Position
0 0 Slave Station
0 1 Master Station not ready to enter logical token ring
1 0 Master Station ready to enter logical token ring
1 1 Master Station in logical token ring
[edit] References
- Profibus at Wikipedia.org
- PROFIBUS Specification: Normative Parts of PROFIBUS -FMS, -DP, -PA according to the European Standard EN 50170 Volume 2
