Technical data

5.2 Data Transmission with HDBs (Free Layer 2
Access)
This type of data transmission is suitable for communication between
SIMATIC S5 programmable controllers and remote PROFIBUS compatible
automation or field devices. SIMATIC S5 programmable controllers can also
communicate with each other with this type of data transmission, although
for this situation, data transmission via S5-S5 links is much easier to handle
(refer to Chapter 7).
With data transmission by direct access to layer 2 services, you can
transmit or receive blocks of data with a length of maximum 242 bytes.
This type of communication has the following characteristics:
➣ The data transmission is by direct access to layer 2 services after being
triggered by an HDB in the PLC program.
➣ The station relationship is a 1 : 1 relationship or a 1 : n relationship
between stations: station (PLC<->CP)----->(CP<->PLC) or non-S5
device station.
➣ The data structures to be transmitted are related blocks of data of 1 to
242 bytes.
➣ The PLC cycle time load as transmitter and receiver is high compared
with implicit communication (GP/ZP/DP).
➣ The L2 service used is SDA, SDN, SRD, RPL_UPD_S, RPL_UPD_M.
Depending on the type of service selected, various security
mechanisms accompany the data transmission.
➣ The priority of the L2 frame is selectable (L (low) / H (high)).
Selecting the Type of Communication B8976060/02
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