Tables 2.1 Tbc Private Area; Next Station Address; Data Organization In Memory; Tbc Private Area - Motorola MC68824 User Manual

SECTION 2
TABLES
The MC68824 Token Bus Controller (TBC) utilizes two tables to interface with the host. The tables
include the TBC private area and the initialization table. The TBC private area is a 128-byte block
of memory used by the TBC to store internal variables and pointers. The initialization table is a
256-byte block of memory which holds initial parameters, interrupt status and interrupt masks, a
DMA dump area, statistical information, and the command parameter area (CPA). The CPA is
used by the host to set and read TBC parameters.
2.1 TBC PRIVATE AREA
The TBC private area is a 128-byte area of RAM reserved for use by the TBC to store internal
variables and statistical information associated with the media access control (MAC) operation.
During initialization, the host CPU specifies the appropriate initial values of the private area
parameters in the initialization table (see displacement 08 hex through 76 hex in the initialization
table shown in Figure 2-3). These initial values are then loaded into the private area through the
INITIALIZE command (see 3.2.2 INITIALIZE Command for details). After initialization, the TBC
keeps an on-chip pointer to this private area. The private area should never be directly accessed
by the host as this would not guarantee IEEE 802.4 operation. The parameters in the private area
should only be set and read by the SET/READ VALUE commands through the command parameter
area (see 3.5 SET/READ VALUE).
The format of the TBC private area is shown in Figure 2-2. Those parameters specified in the IEEE
802.4 standard are noted. All timers are specified in octet times; one octet time is defined as 8-1/
(network data rate)). The data is organized following the Motorola byte ordering convention unless
otherwise specified. That is, the low-order byte has an odd address while the high-order byte has
an even address as shown in Figure 2-1.
D e B A
8 7
Byte 0 [Word 01 Byte 1 High Word
I
MSB
~--------------------------------------------------------~
Low Word
Byte 2 [Word
11
Byte 3
LSB
I
Figure 2·1. Data Organization in Memory
2.1.1 Next Station Address
Next station (NS) address is the 48-bit (or 16-bit) address of the next station in the logical ring
that this station will pass the token to. The address length of 48 or 16 bits for the station is chosen
at initialization time by setting or resetting a bit located in offset 7 A of the initialization table. The
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