Siemens SIMATIC TIWAY 1 - Manual (Page 189)

USING THE BDLC PROTOCOL
UNILINK HOST ADAPTER B–4
USER MANUAL
B.2.2 Address Field
The address field is eight bits in length and specifies the address of the
UNILINK Host Adapter which is to receive the message. Up to 32 adapters may
be multidropped on the same host network as discussed in Paragraph 3.2.3. The
address is a unique value ranging from 0 to 31 and is set on the adapter with
dipswitches as described in Paragraph 3.3.1.1. There is no global address that
causes all the adapters in a multipoint circuit to respond.
B.2.3 Control Field
The control field is an eight bit field containing the BDLC codes for commands
and responses that control transmission, reception, and error recovery of
messages from the host computer to the UNILINK Host Adapter. Table B–1 lists
the minimum command set supported by this protocol. These commands are
described in more detail in Paragraph B.3.
Table B–1 BDLC Protocol Command Set
Host Computer
Command Transmission
I
RR
SNRM
DISC
UNILINK Host Adapter
Response Transmission
I or RR
I or RR
UA
UA
or RNR
or RNR
___________________ ___________________
B.2.4 Information Field
The 290-byte information field is divided into three subfields; the two-byte
length subfield, the one-byte UNILINK Host Adapter command code subfield,
and up to 287 command parameter data bytes. The length subfield contains the
hexadecimal count of the number of bytes in the information field. The smallest
information field will contain the length subfield and the command code
subfield for a minimum count of three or 0003. Inserted DLEs are not included
in the length count. The exception to this format is given in the discussion on the
FRMR response in Paragraph B.5.7.