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II
2.1.14 RX Frame Status Error Mask
If a frame addressed to the TBC is received with one of the errors listed below and the corre-
sponding error mask bit is set to one, the TBC will accept the frame (for more details see 4.1.1.2
RECEIVE STATUS WORD). The format of the status error mask is shown below. This parameter
is initialized by the host through the initialization table and may be altered or read via the SETI
READ VALUE command thereafter.
Offset
26
F E D
C
B
A
4
I CRCE I EBIT I FOVR I NOISE I FTL I NOBUFI
0
I
o
I
0
I
0
I
0
FTL
Frame Too Long
(>8
K)
o
I
0
I
0
I
0
CRCE
EBIT
FOVR
NOISE
CRC Error
E-Bit Error
FIFO Overrun
Noise
NOBUF Not Enough Buffers in Free Buffer Pool
2.1.15 TX Queue Access Class Status
Each transmit queue has an associated transmit queue access class status word. Each status word
is initialized by the host through the initialization table and may be altered or read via the SETI
READ VALUE command thereafter. In each of the access class status words, bit F (aD) contains
the queue disabled bit and bit E (aE) contains the queue empty bit. In order to transmit, the host
processor may set the queue disabled bit to zero to enable the queue and the empty bit to zero
indicating the queue contains data. This action is not necessary since the START command sets
both bits (aD and aE) of the status word to zero, indicating that the queue is enabled and full.
When the TBC is finished transmitting from the queue, it sets the queue empty bit to one. A
transmit queue, except for class 6 frames, must be enabled at least one token rotation time before
it can contain data or before the station enters the logical ring. This is done so the token rotation
timers for each access class have a valid value.
Offset
F E D
C
B
A
9
4
28, 30, 38, 40 I QO I QE I
0
I
0
I
0
I
0
I
0
I
0
I
0
I
o
I
0
I
0
o
I
I
0
QD - Queue Disabled
QE - Queue Empty
1 Queue is Disabled
o
Queue is Enabled
2.1.16 TX Queue Access Class Pointers
1 Queue is Empty
o
Queue is Not Empty
The four transmit queue access class pointers point to the first frame descriptor in the respective
access class queue. These pointers direct the TBC to the location from which to start transmitting.
Each pointer is initialized by the host through the initialization table and may be altered or read
via the SETIREAD VALUE command thereafter. For more details on the management of these
pointers refer to SECTION 4 BUFFER STRUCTURES. The format is shown below.
Offset
o
C
B
A
4
2A, 32, 3A, 42 MSB
Tx Queue Access Class HOQ Pointer - High Word
~----------------------------------------------------------~
2C, 34, 3C, 44
Tx Queue Access Class HOQ Pointer - Low Word
LSB
~----------------------------------------------------------~
2.1.17 RX Queue Access Class Pointers
The four receive queue access class pointers point to the last frame descriptor in the respective
access class queue. Each pointer is initialized by the host through the initialization table and may
MOTOROLA
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MC68824 USER'S MANUAL
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