AM79C983A Advanced Micro Devices, AM79C983A Datasheet - Page 24

• Manufacturer Part Number: AM79C983A
• Description: Integrated Multiport Repeater 2 (IMR2)
• Manufacturer: Advanced Micro Devices

indicate that it is active and is ready to drive the DAT
and JAM signals. The external arbiter asserts ACK if
one and only one IMR2 device has REQ asserted. This
allows the corresponding IMR2 device to drive the DAT
line with data to be repeated by all other IMR2 devices.
If there is more than one IMR2 device asserting REQ,
the external arbiter should assert COL, indicating mul-
tiple IMR2 devices are active.
The active IMR2 device drives the JAM line HIGH in
order to signal other IMR2 devices that it has detected
a collision across one or more of its ports and is gener-
ating a Jam Sequence. The DAT line is used during sin-
gle IMR2 device collision (JAM asserted) to signal
single-port collision (DAT HIGH) or multiport collision
(DAT LOW). Other IMR2 devices synchronize their in-
ternal Collision Jam Sequence generators using JAM
and DAT pins as inputs.
If more than one IMR2 device is active (multiple REQs
asserted), the external arbiter should assert the COL
line to signal this condition. In this case, all IMR2 de-
vices in the repeater are forced into the multiport colli-
s i o n s t a t e a n d w i l l g e n e ra t e Ja m s e q u e n c e
independently while this condition lasts. As ports on
separate IMR2 devices back off, the last IMR2 device
with an active port regains control of the DAT and JAM
signals and all other IMR2 devices will continue gener-
ating Jam sequence while the JAM signal is asserted.
In a typical single-board application, three IMR2 de-
vices can be connected together without the use of ex-
ternal transceivers. The total number of IMR2 devices
that can be used in a more complex architecture will
depend on the drive capability, system timing limita-
tions, and system design.
The external arbiter is required to generate two signals
(ACK and COL). The logic function for these signals in
a three IMR2 device Repeater Unit is as follows:
ACK = REQ1 & !REQ2 & !REQ3 + !REQ1 & REQ2 &
!REQ3 + !REQ1 & !REQ2 & REQ3
COL = !(ACK + !REQ1 & !REQ2 & !REQ3)
Asynchronous Mode Operation
The operation of the expansion bus in the asynchro-
nous mode is similar to the operation in the synchro-
nous mode. The primary difference is that the clock
signal in the asynchronous mode is ECLK, which is
sourced by the IMR2 device transmitting DAT. The sig-
nals JAM, REQ, ACK, and COL are all asynchronous.
DAT is synchronized to ECLK, which is a 10-MHz clock
signal. When the IMR2 device asserts REQ and re-
ceives an ACK, ECLK is an output. When the IMR2 de-
vice does not assert REQ and receives an ACK, ECLK
is an input.
In the asynchronous mode, it is probable that ECLK and
the master clocks of the receiving IMR2 devices will be
24
P R E L I M I N A R Y
Am79C983A
FRAME
skewed in frequency. To help the IMR2 devices accom-
modate the frequency differences, the expansion bus
transmits a framing signal (FRAME). See Figure 2.
Because JAM is an asynchronous signal, there is no
defined relationship between JAM and ECLK.
Packet Statistics
Packet Report Port
For each packet, the IMR2 device can compile a set of
data about that packet. This data, which will now be re-
ferred to as the report packet, allows the system to de-
rive objects in the Host, HostTopN, and Matrix groups
of the RMON MIB (RFC 1757). The Report Packet is
delivered by the Packet Report Port (PR).
The PR port transmits a portion of the packet along
with data about that packet to a MAC. The format of the
report packet is shown in 3. Sending only a portion of
the packet is referred to as packet compression.
The degree to which the original packet is compressed
is set by the Report Packet Size Register. The size is in
bytes. If the register is set to 14 or less, the size of the
packet passed is 14 bytes. If the register is set to 1536
or greater, the entire packet is passed. If the packet
size is equal to or less than the value set in the Report
Packet Size Register, the entire packet is passed.
If the destination address of the packet is the same as
the address of the MAC connected to the PR Port, then
it is desirable to have the entire packet transmitted to
the MAC. Therefore, packet compression is automati-
cally disabled when the destination address of the
packet is a valid address for the expansion bus. How-
ever, the report tag is appended to the end of the
packet. Note that the entire packet is also sent if the
destination address is a broadcast address.
ECLK
ACK
DAT
JAM
Figure 2. Asynchronous Mode Data Transfer
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