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=== Data link layer (OSI-Layer 2) === The data link layer '''FDL''' (Field bus Data Link) services<ref name=iec61158-3-3/>and protocols<ref name=iec61158-4-3/> work with a hybrid access method that combines [[token passing]] with a master/slave method. In a PROFIBUS DP network, the controllers or process control systems are the masters and the sensors and actuators are the slaves.<ref name=PI4072/><ref name=felser /> Each byte has even parity and is transferred asynchronously with a start and stop bit. There may not be a pause between a stop bit and the following start bit when the bytes of a telegram are transmitted. The master signals the start of a new telegram with a SYN pause of at least 33 bits (logical "1" = bus idle). Various telegram types are used. They can be differentiated by their start [[delimiter]] (SD): ==== No data ==== SD1 = 0x10 {| border="1" |bgcolor="#ffff99"|SD1 |bgcolor="#ffcc99"|DA |bgcolor="#ffcc99"|SA |bgcolor="#77ffff"|FC |bgcolor="#ff7777"|FCS |bgcolor="#ffff99"|ED |} ==== Variable length data ==== SD2 = 0x68 {| border="1" |bgcolor="#ffff99"|SD2||LE||LEr||bgcolor="#ffff99" |SD2 |bgcolor="#ffcc99"|DA |bgcolor="#ffcc99"|SA |bgcolor="#77ffff"|FC |bgcolor="#99ff99"|DSAP |bgcolor="#99ff99"|SSAP ||PDU |bgcolor="#ff7777"|FCS |bgcolor="#ffff99"|ED |} ==== Fixed length data ==== SD3 = 0xA2 {| border="1" |bgcolor="#ffff99"|SD3 |bgcolor="#ffcc99"|DA |bgcolor="#ffcc99"|SA |bgcolor="#77ffff"|FC ||PDU |bgcolor="#ff7777"|FCS |bgcolor="#ffff99"|ED |} ==== Token ==== SD4 = 0xDC {| border="1" |bgcolor="#ffff99"|SD4 |bgcolor="#ffcc99"|DA |bgcolor="#ffcc99"|SA |} ==== Fields ==== {| class="wikitable" border="1" | '''SD''' | Start Delimiter |- | '''LE''' | Length of protocol data unit, (incl. DA, SA, FC, DSAP, SSAP) |- | '''LEr''' | Repetition of length of protocol data unit, (Hamming distance = 4) |- | '''FC''' | Function Code |- | '''DA''' | Destination Address |- | '''SA''' | Source Address |- | '''DSAP''' | Destination [[Service Access Point]] |- | '''SSAP''' | Source [[Service Access Point]] |- | '''PDU''' | Protocol Data Unit (protocol data) |- | '''FCS''' | [[Frame check sequence|Frame Checking Sequence]], calculated by simply adding up the bytes within the specified length. An overflow is ignored here. |- | '''ED''' | End Delimiter (= 0x16) |} ==== Service Access Points ==== {| class="wikitable" border="1" ! SAP (Decimal) ! SERVICE |- | Default 0 | Cyclical Data Exchange (Write_Read_Data) |- | 54 | Master-to-Master SAP (M-M Communication) |- | 55 | Change Station Address (Set_Slave_Add) β SAP55 is optional and may be disabled if the slave doesn't provide non-volatile storage memory for the station address. |- | 56 | Read Inputs (Rd_Inp) |- | 57 | Read Outputs (Rd_Outp) |- | 58 | Control Commands to a DP Slave (Global_Control) |- | 59 | Read Configuration Data (Get_Cfg) |- | 60 | Read Diagnostic Data (Slave_Diagnosis) |- | 61 | Send Parameterization Data (Set_Prm) |- | 62 | Check Configuration Data (Chk_Cfg) |}
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