Message Frames; Bit Arbitration Example - Intel 87C196CA Supplement To User’s Manual

8XC196L X SUPPLEMENT
of arbitration, nodes A, C, and D are all transmitting an "active 0" symbol, thus the idle state of
the "passive 1" symbol is overruled in favor of the driven state of the "active 0" symbol.
Node C is the next node to discontinue transmitting when it attempts to take control of the bus by
transmitting an "active 1" symbol. However, nodes A and D maintain control by continuing to
drive the bus with an "active 0" symbol.
Finally, node D discontinues transmitting when its attempt to hold the bus in an idle state is over-
ruled by the driven state of the "active 1" symbol on node A. Thus, node A is awarded arbitration.
The busline signal, detected on the bus by the receiver, reflects node A's message, as this is the
only node that did not experience contention.
"0" "0"
Node A
"0" "0"
Node B
"0" "0"
Node C
"0" "0"
Node D
Busline
8.4

MESSAGE FRAMES

A message transmission or reception is transferred within a message frame that adds control and
error-detection bits to the content of the message. The frame for an IFR message differs slightly
from that for a standard message, but they contain similar information (Figure 8-6).
8-8
"0" "0"
"1"
Point of
Arbitration
"0" "0"
"0" "0"
Figure 8-5. Bit Arbitration Example
"1" "0"
Point of
"1" "1"
Arbitration
"1" "0"
"0" "1"
"1"
Point of
Arbitration
A5223-01