Siemens SIMATIC TIWAY 1 - Manual (Page 186)

UNILINK HOST ADAPTER B–1
USER MANUAL
APPENDIX B
USING THE BDLC PROTOCOL
The BDLC protocol is a subset of the ANSI X3.66–1979 Advanced Data
Communication Control Procedure (ADCCP). BDLC provides a data link
between the host computer and one or more UNILINK Host Adapters. The data
link is responsible for establishing and terminating logical connections between
the host computer and the adapters, to handle data transfers between them,
and to ensure message integrity in those transfers. The most significant
difference between BDLC and the ADCCP standard is that ADCCP frames are
transmitted as a continuous stream of data bits, but BDLC transmits frames as
a series of data bytes allowing the use of common serial communications devices
through the RS–232–C/RS–423–A host port, Port 3. In other words, BDLC is an
asynchronous data link protocol. The physical link layer protocol can use either
asynchronous or synchronous operation as selected by the dipswitch settings
described in Chapter 3.
The main requirements for data link control are
1. Bracket the information being sent to distinguish the information from
other (noise) signals.
2. Identify and permit addressing of the receiver.
3. Detect errors and provide a recovery method.
4. Manage the sharing of the link among multiple station.