beautypg.com

B.3 bdlc protocol control field structures, Using the bdlc protocol – Siemens SIMATIC TIWAY 1 User Manual

Page 191

background image

USING THE BDLC PROTOCOL

UNILINK HOST ADAPTER

B–6

USER MANUAL

B.3

BDLC PROTOCOL CONTROL FIELD STRUCTURES

P/F

N(S)

0

N(R)

P/F

1

N(R)

P/F

1

N

0

1

2

3

4

5

6

7

MSB

LSB

0

1

S

M

I–Frame Format

Supervisoty Frame

Unnumbered Frame

Format

Format

First bit transmitted

Where: N(S) is the transmitting station send sequence number.

N(R) is the transmitting station recieve sequence number.
S
M
P/F
N

is the supervisory function bits.
is the modifier function bits.
is the Poll/Final bit.
is the modifier function bits.

Figure B–4

BDLC Protocol Control Field Structures

The control field will be one of three formats as shown in Figure B–4. The
I-Frame format is used to transfer messages between the host computer and the
adapter. The N(S) and N(R) sequence numbers are Modulo–8 counts. These
counts begin at zero when the host computer issues the Set Normal Response
Mode (SNRM) command to link the adapter. The counts are incremented by one
for each frame transmitted in either direction.

The Supervisory frame format is used to perform link supervisory control
functions such as acknowledge I-Frames, request retransmission of an
I-Frame, and others. Responses of this type do not contain the Information Field
and only update the N(R) sequence count. Valid S subfield values are:

00

RR – Receive ready

01

RNR – Receive not ready

10

REJ – Reject