am79c981 Advanced Micro Devices, am79c981 Datasheet - Page 16

• Manufacturer Part Number: am79c981
• Description: Integrated Multiport Repeater Plus? Imr+? ??9
• Manufacturer: Advanced Micro Devices

SET (Write) Opcodes
IMR+ Chip Programmable Options
SI data:
SO data:
IMR+ Chip Programmable Options can be enabled (dis-
abled) by setting (resetting) the appropriate bit in the
command string. The three programmable bits are:
C—CI Reporting; S—AUI SQE Test Mask, and
A—Alternative Port Activity Monitor (PAM) Function.
These options can be enabled (disabled) by setting (re-
setting) the appropriate bit in the command string.
When writing to this register through the Am79C987
HIMIB device, the A and C bits should not be changed
(A=0, C=1).
C—CI Reporting
Setting this bit alters the function of the STR pin. In this
mode, the STR pin becomes an input in response to the
AMD’s Am79C987 HIMIB device. Upon deassertion of
RST, the HIMIB automatically sets this bit following IMR/
IMR+ device type detection.
When this mode is selected, the CRS output bit string
format is modified to include CI carrier bit (in addition to
AUI carrier). This bit occupies the bit position immedi-
ately preceding the AUI bit in the CRS bit string (10 bits)
output. Note that the AUI bit gets asserted if either the CI
or DI signal pairs are active.
S—AUI SQE Test Mask
Setting this bit allows the IMR+ chip to ignore activity on
the CI signal pair, in the SQE Test Window, following a
transmission on the AUI port. This event occurs when
the attached MAU has the SQE Test option enabled,
therefore generating a burst of CI activity following every
transmission. This is interpreted by the IMR+ device as
a collision, causing the IMR+ device to generate a full
Jam pattern. Although the MAU attached to a repeater is
required not to have its SQE test function active, this is a
common installation error, causing difficulty in diagnos-
ing network throughput problems.
The SQE Test Window, as defined by the IEEE 802.3
(Section 7.2.2.2.4), is from 6-bit times to 34-bit times
(0.6 s to 3.4 s). This includes delay introduced by a
50 m AUI. CI activity that occurs outside this window is
not ignored and is treated as true collision.
Note that enabling this function does not prevent the re-
porting of this condition by the IMR+ device and the two
functions operate independently.
1–86
AMD
0000 1CSA
None
PRELIMINARY
Am79C981
A—Alternative Port Activity Monitor (PAM)
Function
Setting the Alternative Port Activity Monitor Function al-
lows the PAM function to be altered such that the Carrier
Sense data is presented unmodified. In default opera-
tion the PAM output (Carrier Sense bits in the CRS bit
stream) are masked if the port is either disabled or parti-
tioned. This does not allow the Repeater Management
software to sense activity on all segments at all times.
The ability to monitor partitioned or disabled ports allows
fault tolerance to be built into the Repeater Management
software.
Alternate AUI Port Partitioning Algorithm
The AUI port Partitioning/Reconnection scheme can be
programmed for the alternate (transmit only) reconnec-
tion algorithm by invoking this command. To return the
AUI back to the standard (transmit or receive)
reconnection algorithm, it is necessary to reset the
IMR+ device. Standard partitioning algorithm is se-
lected upon reset.
Alternate TP Ports Partitioning Algorithm
The TP ports Partitioning/Reconnection scheme can be
programmed
reconnection algorithm by invoking this command. All
TP ports are affected as a group by this command. To
return the TP ports back to the standard (transmit or re-
ceive) reconnection algorithm, it is necessary to reset
the IMR+ device. The standard partitioning algorithm is
selected upon reset.
AUI Port Disable
The AUI port will be disabled upon receiving this com-
mand. Subsequently, the IMR+ chip will ignore all inputs
(Carrier Sense and SQE) appearing at the AUI port and
will not transmit any data or Jam Sequence on the AUI
port. Issuing this command will also cause the AUI port
to have its internal partitioning state machine forced to
its idle state. Therefore, a Partitioned Port may be re-
connected by first disabling and then re-enabling
the port.
SI data:
SO data:
SI data:
SO data:
SI data:
SO data:
for
the
00011111
None
00010000
None
00101111
None
alternate
(transmit
only)