Siemens SPC3 Specifications download pdf (Page 15)

Introduction To ProfiBus DP

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Acromag, Inc. Tel:248-624-1541 Fax:248-624-9234 Email:sales@acromag.com http://www.acromag.com

ProfiBus networks employ the

even parity

method of data error

checking which controls how the parity bit of a data frame is set.

With even parity checking, the number of 1 bits in the data portion of

each character frame is counted. Each character contains 8 bits. The parity

bit will then be set to a 0 or 1, as required in order to result in an even total

number of 1 bits. For example, if a character frame contains the following

eight data bits: 1100 0011, then since the total number of 1 bits is 4 (already

an even number), the frame’s parity bit will be set to 0 for even parity.

When a message is transmitted, the parity bit is calculated and applied

to the frame of each character transmitted. The receiving device counts the

quantity of 1 bits in the data portion of the frame and sets an error flag if the

count differs from that sent. As such, parity checking can only detect an

error if an odd number of bits are picked up, or dropped off, from a character

frame during transmission. For example, with even parity, if two 1 bits are

dropped from a character, the result is still an even count of 1 bits and no

parity error will be detected.

A ProfiBus telegram may contain up to 256 bytes--up to 244 bytes of

data per node per message, plus 11 bytes of overhead. This overhead is

referred to as the

Telegram Header

. All telegram headers are 11 bytes,

except for Data_Exchange telegrams which have 9 bytes of header

information (the DSAP and SSAP bytes are dropped). Note that 12 bytes is

a lot of overhead for a single message and this makes ProfiBus less efficient

for small amounts of data. However, since up to 244 bytes of data may

occur per message, and since the output data is sent and the input data

received in a single telegram cycle, this makes ProfiBus more efficient when

large amounts of data must be transferred. Note that an idle state of at least

33Tbits (sync-time in bit time) must be present before every request

telegram to be sent, and all data is transferred without gaps between

individual characters. All data exchanges between a Master and Slave are

handled in the telegram header using Service Access Points (SAP’s).

ProfiBus DP uses SAP’s 54 to 62, plus the default SAP (Data_Exchange).

Telegram Header Data and Frame

SD LE LEr SD DA SA FC DSAP SSAP DU FCS ED

1b 1b 1b 1b 1b 1b 1b 1b 1b var 1b 1b

ProfiBus Data Error

Checking

PROFIBUS TELEGRAM

(MESSAGE)

STRUCTURE

1 2 ... 10 11 12 13 14 15 16 17 18 19 20 ... 37 38

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Siemens SPC3 Specifications Page 15

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