TABLE 9- SETUP REGISTER
BIT
BIT NAME
SYMBOL
DESCRIPTION
7
Pulse1 Mode
P1MODE
This bit determines the type of PULSE1 output driver used in
Backplane Mode. When high, a push/pull output is used. When
low, an open drain output is used. The default is open drain.
6
5
4
Four NACKS
FOUR
NACKS
This bit, when set, will cause the EXNACK bit in the Diagnostic
Status Register to set after four NACKs to Free Buffer Enquiry are
detected by the COM20020-5. This bit, when reset, will set the
EXNACK bit after 128 NACKs to Free Buffer Enquiry. The default
is 128.
ET3
ET3
This bit, when set, scales down protocol timeout values of
Response Time and Idle Time but NOT Reconfiguration Time to
optimize network performance in short topologies. Provides a
scaling factor of
3. Defaults to a zero. Must be reset to be
ARCNET compliant.
Receive All
RCVALL
This bit, when set, allows the COM20020-5 to receive all valid data
packets on the network, regardless of their destination ID. This
mode can be used to implement a network monitor with the
transmitter on- or off-line. Note that ACKs are only sent for packets
received with
a destination ID equal to the COM20020-5's
programmed node ID. This feature can be used to put the
COM20020-5 in a 'listen-only' mode, where the transmitter is
disabled and the COM20020-5 is not passing tokens. Defaults low.
3,2,1 Clock Prescaler Bits
CKP3,2,1
These bits are used to determine the data rate of the COM20020-5.
3,2,1
The following table is for a 40MHz crystal:
CKP3
CKP2 CKP1
DIVISOR
SPEED
5Mbs
2.5Mbs
1.25Mbs
625Kbs
312.5Kbs
Reserved
Reserved
Reserved
8
16
32
64
128
256
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
NOTE: The lowest data rate achievable by the COM20020-5 is
312.5Kbs. A divide by 256 is provided for those systems that use
faster clock speeds. Defaults to 000 or 5Mbs.
0
Slow Arbitration Select SLW-ARB
This bit, when set, will divide the arbitration clock by 2. Memory
cycle times will increase when slow arbitration is selected. The
SLOWARB bit must be set for 5Mbs operation.
NOTE: For clock speeds greater than 20MHz, SLOWARB must be
set. Defaults to low.
28
SMSC [ SMSC CORPORATION ]
