am79c981 Advanced Micro Devices, am79c981 Datasheet - Page 11

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

Expansion Port
The IMR+ chip Expansion Port is comprised of five pins;
two are bi-directional signals (DAT and JAM), two are in-
put signals (ACK and COL), and one is an output signal
(REQ). These signals are used when a multiple-IMR+
device repeater application is employed. In this configu-
ration, all IMR+ chips must be clocked synchronously
with a common clock connected to the X1 inputs of all
IMR+ devices. Reset needs to be synchronized to
X1 clock.
The IMR+ device expansion scheme allows the use of
multiple IMR+ chips in a single board repeater or a
modular multiport repeater with a backplane architec-
ture. The DAT pin is a bidirectional I/O pin which can be
used to transfer data between the IMR+ devices in a
multiple-IMR+ chip design. The data sent over the DAT
line is in NRZ format and is synchronized to the common
clock. The JAM pin is another bidirectional I/O pin that is
used by the active IMR+ chip to communicate its internal
status to the remaining (inactive) IMR+ devices. When
JAM is asserted HIGH, it indicates that the active IMR+
device has detected a collision condition and is generat-
ing Jam Sequence. During this time when JAM is as-
serted HIGH, the DAT line is used to indicate whether
the active IMR+ chip is detecting collision on one port
only or on more than one port. When DAT is driven
HIGH by the IMR+ chip (while JAM is asserted by the
IMR+ chip), then the active IMR+ device is detecting a
collision condition on one port only. This ‘one-port-left’
signaling is necessary for a multiple-IMR+ device re-
peater to function correctly as a single multiport repeater
unit. The IMR+ chip also signals the ‘one port left’ colli-
sion condition in the event of a runt packet or collision
fragment; this signal will continue for one expansion port
bus cycle (100 ns) before deasserting REQ.
The arbitration for access to the bussed bi-directional
signals (DAT and JAM) is provided by one output (REQ)
and two inputs (ACK and COL). The IMR+ chip asserts
the REQ pin to indicate that it is active and wishes to
drive the DAT and JAM pins. An external arbiter senses
the REQ lines from all the IMR+ devices and asserts the
ACK line when one and only one IMR+ chip is asserting
its REQ line. If more than one IMR+ chip is asserting its
REQ line, the arbiter must assert the COL signal, indi-
cating that more than one IMR+ device is active. More
PRELIMINARY
Am79C981
than one active IMR+ device at a time constitutes a colli-
sion condition, and all IMR+ devices are notified of this
occurence via the COL line of the Expansion Port.
Note that a transition from multiple IMR+ devices arbi-
trating for the DAT and JAM pins (with COL asserted,
ACK deasserted) to a condition when only one IMR+
chip is arbitrating for the DAT and JAM pins (with ACK
asserted, COL deasserted) involves one expansion port
bus cycle (100 ns). During this transitional bus cycle,
COL is deasserted, ACK is asserted, and the DAT and
JAM pins are not driven. However, each IMR+ device
will remain in the collision state (transmitting jam se-
quence) during this transitional bus cycle. In subse-
quent expansion port bus cycles (REQ and ACK still
asserted), the IMR+ devices will return to the ‘master
and slaves’ condition where only one IMR+ device is ac-
tive (with collision) and is driving the DAT and JAM pins.
An understanding of this sequence is crucial if non-
IMR+ devices (such as an Ethernet controller) are con-
nected to the expansion bus. Specifically, the last
device to back off of the Expansion Port after a multi-
IMR+ chip collision must assert the JAM line until it too
drops its request for the Expansion Port.
External Arbiter
A simple arbitration scheme is required when multiple
IMR+ devices are connected together to increase the to-
tal number of repeater ports. The arbiter should have
one input (REQ1...REQn) for each of the n IMR+ de-
vices to be used, and two global outputs (COL and
ACK). This function is easily implemented in a PAL de-
vice, with the following logic equations:
ACK =
COL =
Above equations are in positive logic, i.e., a variable is
true when asserted.
A single PALCE16V8 will perform the arbitration func-
tion for a repeater based on several IMR+ devices.
+
+
REQ1 & REQ2 & REQ3 & ....REQn
REQ1 & REQ2 & REQ3 & ....REQn
REQ1 & REQ2 & REQ3 & .... REQn
ACK & (REQ1 + REQ2 + REQ3 + ... REQn)
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