Motorola MC68302 User Manual page 199

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NOTE
The DDCMP controller can transmit ones in both NAZ and NAZI data
encoded formats.
1 =Send SYN1-SYN2 pairs between messages. ATS is always asserted.
Note that SYN1 and SYN2 may be the same character.
ENC - Data Encoding Format
O
=Non return to Zero (NAZ).
A
one is a high level; a zero is a low level.
1 = Nonreturn to Zero Inverted (NAZI).
A
one is represented by no change
in the level; a zero is represented by a change in the level.
4.5.14.10 DDCMP RECEIVE BUFFER DESCRIPTOR (Rx BO). The CP reports infor-
mation about the received data for each buffer using the BDs. The Rx BO is
shown in Figure 4-17. The CP closes the current buffer, generates a maskable
interrupt, and starts to receive data in the next buffer after any of the following
events:
OFFSEhO
OFFSET+ 2
OFFSET+ 4
OFFSET+B
4-96
Receiving the received message length number of bytes (RMLG)
Detecting an error
Detecting a full receive buffer
Issuing the ENTER HUNT MODE command
The first word of the Rx BO contains control and status bits. Bits 15-12 are
written by the user before the buffer is linked to the Rx BO table, and bits
4-0 and 11-8 are set by the IMP following message reception. Bit 15 deter-
mines whether the M68000 core or the CP may currently access the BO.
E- Empty
1 =The data buffer associated with this BO is empty. This bit signifies
that the BO and its associated buffer are available to the DDCMP
controller. The M68000 core should not write to any fields of this BO
when this bit is set. Note that the empty bit will remain set while the
DDCMP controller is currently filling the buffer with received data.
15 14 13 12 11 10
E
l
x
1
w
1
I
1
L
I
H
J
r2
J
Tl
J -
I - I - I
FR
J
PR
J
CR
I
av
I
CD
DATA LENGTH
RX BUFFER POINTER
Figure 4-17. DDCMP Receive Buffer Descriptor
MC68302 USER'S MANUAL MOTOROLA