SN75LBC088

AUI CONCENTRATOR

SLLS150A – DECEMBER 1992 – REVISED MAY 1993

2–2

POST OFFICE BOX 655303

description (continued)

This device also has two operational modes, local and global, and a self-exerciser test mode. The SN75LBC088

uses the LinBiCMOS

™ 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-

Ω, 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.

LinBiCMOS is a trademark of Texas Instruments Incorporated.