Siemens SIMATIC TIWAY 1 - Manual (Page 187)

USING THE BDLC PROTOCOL
UNILINK HOST ADAPTER B–2
USER MANUAL
B.1 BDLC CONFIGURATION
BDLC is an unbalanced configuration meaning that there is one primary
station (the host computer) and one or more secondary stations (UNILINK Host
Adapters) as illustrated in Figure B–1. The primary is responsible for setting
each secondary to the normal response mode and for controlling the information
exchange.
PRIMARY
(Host Computer)
Commands
Responses
Secondary
Adapter
“A”
Secondary
Adapter
“B”
Figure B–1 BDLC Unbalanced Configuration
In the normal response mode, a secondary station may only transmit frames as
a direct result of receiving permission from the primary. The primary grants
this permission by polling the secondary stations.