SN75LBC088 Datasheet, PDF (3/16 Pages) Texas Instruments – AUI CONCENTRATOR

SN75LBC088
AUI CONCENTRATOR
SLLS150A – DECEMBER 1992 – REVISED MAY 1993
description (continued)
In global mode, the local station users are logically connected to the LAN backbone media. Global mode has
two types of signal flow patterns: station to other stations and the LAN, and the LAN to all stations. When a station
starts to transmit, its squelch deactivates and is considered active. The control logic then selects the active
channel’s data for transmission to the LAN. Unlike the local mode, the other stations do not get the data directly
from the active port. Data first reaches the transceiver, gets looped back, and then is sent to the eight STXx
drivers. This action emulates the operation between a station and a transceiver in a normal point-to-point link.
In global mode, local and global collisions are handled differently. For a local collision, the device cannot force
a collision on the LAN backbone directly. To create a collision on the LAN, the device transmits a 5-MHz signal
onto the GTX drivers to force activity on the LAN segment. Any LAN activity collides with this forced 5-MHz signal
and is seen as a collision by the collision receiver. This action keeps the network synchronized. After the global
port’s data loops back from the LAN, the collision signal is sent to all the local nodes via the SCLx output drivers.
A global collision (collision on the network) is handled normally since station transmit data is routed to the GTX
driver. In this instance, data sources are directly in collision. Once a collision is detected on the network, the
transceiver asserts a collision signal that is detected on the GCL input receiver. The GCL receiver collision signal
is then routed to all the SCLx output drivers tied to the stations.
In global mode, the transceiver generates SQE. When a station finishes a transmission, the transceiver
generates the SQE. This is detected on the GCL input. When Smart SQE is enabled (SMARTSQE pulled low),
the SQE is sent to the station that originated the transmission. Because of this activity, the ACC has to remember
which station transmitted the last signal and only allow collision back to that station during the SQE window.
Once the SQE passes, the ACC then allows a collision signal back to all stations to indicate a network collision.
When Smart SQE is disabled, the SQE signal is routed to all station collision lines (SCLx).
The SN75LBC088 supports a self-exerciser test mode. The self-exerciser mode tests all the drivers and
receivers on the chip. This mode is invoked by pulling both GLOBAL and TEST low. While in the self-exerciser
mode, a 6.4-µs packet is generated of consistent preamble on the GTX driver port with a 6.4-µs idle time. The
GTX driver, with the help of loop back connectors, routes the preamble to both the GRX and the GCL receivers.
The GRX data is then sent internally to all the STXx drivers. External connectors on the STXx drivers individually
loop this data back to the local SRXx receiver. When the squelch for a receiver is turned off and the global GCL
receiver is unsquelched, the collision driver for that receiver starts sending a collision signal. Each port drives
a collision signal based on its own SRXx receiver squelch being held high and the presence of a global collision
signal, therefore exercising all the drivers and receivers on the chip.
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