SLLS150A - DECEMBER 1992 - REVISED MAY 1993
LinBiCMOS is a trademark of Texas Instruments Incorporated.
The SN75LBC088 attachment unit interface (AUI) concentrator chip (ACC) incorporates eight data terminal equipment (DTE) or station ports and one medium attachment unit (MAU) or global port on the same chip for connection to a local area network (LAN). Each station port emulates the driver/receiver functionality, timing, and signal response of a transceiver or MAU designed to meet the IEEE 802.3-1990 standard. The functional components of the ACC are a differential driver, collision detection driver, and a differential line receiver/squelch.
This device also has two operational modes, local and global, and a self-exerciser test mode. The SN75LBC088 uses the LinBiCMOSTM process technology to ensure high-speed operation, analog precision, and low power consumption.
Each of the eight station ports includes two differential drivers (STX1 thru STX8 [STXx] and SCL1 thru SCL8 [SCLx]) and one differential receiver [SRX1 thru SRX8 (SRXx)]. The SRXx (station receive) input pair is for receiving data sent from the station to the network. The STXx (station transmit) output pair is for transmitting network data to the station. The SCLx (station collision) output pair transmits the collision condition to the station.
The global port supports one differential driver (GTX) and two differential receivers (GRX and GCL). The GTX output pair drives data from a station port to the network. The GRX input pair receives network data from the external transceiver and channels it to all eight station ports. The GCL input pair receives network collision status to be forwarded to the individual station ports.
Each station port differential output pair of the SN75LBC088 drives a 78-
, balanced, terminated, twisted-pair transmission line up to 50 meters. In the off or idle state, the drivers maintain minimal differential output voltage on the twisted-pair lines and remain within the required output common-mode range. When the driver is internally enabled, the driver goes through what is called a soft start or half-step driver start up due to the first transition out of idle swings only half the normal differential amplitude. The differential outputs then rise to full amplitude output levels within 35 ns. The output amplitude is maintained for the remainder of the packet. After the last transmitted packet positive edge, the driver's enable circuit maintains the differential potential above the output common-mode voltage for at least 210 ns, decay down to a minimum differential voltage, and then return to an idle state. Each driver powers up in the idle state to ensure no activity is placed on the twisted-pair cable that could be interpreted as network traffic.
The line receiver squelch function interfaces to a differential twisted-pair line terminated external to the device. The receiver squelch circuit allows differential receive signals to pass through while the input amplitude and pulse duration are greater than the minimum squelch threshold. This ensures a good signal-to-noise ratio while the data path is active and prevents system noise from causing false data transitions during line shut-down and line-idle conditions.
The SN75LBC088 functional control logic operates in two externally switched modes, local and global. Depending on the selected mode, the internal control logic selects the proper internal data path routing and collision handling. The internal data path is altered prior to enabling external line drivers to prevent data transmissions occurring during data path multiplexing.
Local mode is the simplest of the two modes of operation. While all SRXx input receivers from the stations are inactive, the device is in an idle state. The idle state disables all the STXx and SCLx output drivers to the stations. While in local mode, all control signals to and from the global port are logically disabled by the control logic. When transmit activity is detected on any of the eight SRXx input receivers, the channel's internal squelch goes high. While this condition exists, the single SRXx receiver is routed to all STXx drivers. When the transmission is complete, the channel's internal squelch returns low. This starts an end-of-packet hold on all the STXx output drivers. The driver switches to the idle state after the hold time has elapsed. During the specified squelch (SQE) test interval, the SN75LBC088 internally generates a SQE test burst. When Smart SQE is enabled (SMARTSQE pulled low), the SQE test burst is sent to the SCLX output of the station that transmitted last. If Smart SQE is not enabled, it sends the burst to all the SCLx outputs.The device recognizes a collision when one station is active and any other station(s) becomes active. The device then places a 10-MHz collision signal on all the SCLx output drivers. All STXx data is considered undefined during a collision. The STXx drivers are shut down while the SCLx drivers are active and are not reactivated until all SRXx receiver activity is finished. The device returns to the idle state after all transmit traffic has ceased.
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-us packet is generated of consistent preamble on the GTX driver port with a 6.4-us 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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