Motorola MC68824 User Manual page 144

TOKEN BUS
TBC 0036
STATION 0030 STATION 0032 STATION 0034
The IA mask is not applied to control frames and the IA mask least significant bit should always
be zero. If the IIG bit is zero indicating individual addressing, the following opertion is performed
to determine whether to accept the frame:
DA AND IA Mask=This Station Address (TS) AND IA Mask
Some examples are shown below. All examples have this station address (TS) equal to 0036 hex
and an address length of 16 bits.
To accept only data from this station (DA=TS), the IA mask is set as below:
IA Mask= 1111 1111 1111 1110 (FFFE hex)
To accept data frames that are for one of the four stations 0030, 0032, 0034, or 0036:
IA Mask= 1111 1111 1111 1000 (FFF8 hex)
To cite a specific example, if the destination address of an incoming frame is set to 0032, the
operation for acceptance of the frame is performed by the TBC as follows:'
0032 (DA) ANDed with FFF8 (lA Mask) = 0036 (TS) ANDed with FFF8 (lA Mask)
Since the equation is true, the frame is accepted. A binary representation of the acceptance
equation is shown below:
DA
=0000 0000 0011 0010
IA Mask
= 1111 1111 1111 1000
DA AND IA Mask:
=0030 hex
IA Mask =0000 0000
0011 0000
TS =0000 0000
0011 0110
IA Mask = 1111 1111 1111
1000
TS AND IA Mask:
=0030 hex
TS
=0000 0000 0011 0110
The equation would also be true if DA=0030, 0031, 0033, 0034, 0035, and 0036 so frames with
these DAs would also be accepted.
To accept data frames that are only for one of the two stations 0036, 0136:
IA Mask = 1111 1110 1111 1110 (FEFE hex)
MC68824 USER'S MANUAL MOTOROLA
8-5
II