AM79C981 datasheet(16/40 Pages) AMD

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AMD

PRELIMINARY

1–86

Am79C981

SET (Write) Opcodes

IMR+ Chip Programmable Options

SI data:

0000 1CSA

SO data:

None

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

µs to 3.4 µs). 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.

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

SI data:

00011111

SO data:

None

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

SI data:

00010000

SO data:

None

The TP ports Partitioning/Reconnection scheme can be

programmed

for

the

alternate

(transmit

only)

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

SI data:

00101111

SO data:

None

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.

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