SMSC DS – TMC2005

Page 11

Rev. 10/24/2000

OPERATIONAL DESCRIPTION

Direction Determination

All TX ports are set to disable mode in the initial state. When a signal is received from any RX ports, the circuit

holds the port on receiving mode (disable TX) and changes the other ports to sending mode (disable RX). One

port stays in RX and the rest change into TX after all. The circuit initializes the internal DPLL on the timing of

received RX pulse, and the RX buffer circuit stores the RX data and filters its jitter. TX controlling circuit regenerates

the stored RX pulse on nPULSE1, nPULSE2 and nP1BAK. The nPULSE1 and nPULSE2 are pulse output pins for

transceivers (HYC9068S-SK/9088S-SK) of ARCNET normal mode. The nP1BAK is a pulse output pin for

transceiver (HYC2485S/2488S and RS485 driver) of ARCNET back plane mode. When using optical transceiver,

instead of these signals, TXENA [0:1], TXENB [0:1], EXTTX (MA, MB, ME = 0) must be used as TX data inputs of the

optical transceiver.

Direction Release

On ARCNET protocol, each TX message starts with 6-bits of “1” ALERT and each data byte is lead by three bits (1,

1, 0) preamble. To control the HUBs direction, the circuit monitors this bit pattern and holds the state. If the end of

the bit pattern comes, all TX ports return receiving mode (disable TX) again. The interval timer detects the end of

the bit pattern. During data is on line, silent period is less than 4 uS* because at least one bit “1” among 10-bits is

received while receiving the data. The minimum silent interval from the end of received data to the alert of the next

data (the minimum time of changing the direction) is the chip turn around time (12.6 uS*) of ARCNET controller.

The interval timer to detect the data end is set to 5.6uS by adding some margin to the above interval for neglecting

the reflection on a cable.

[Note] Numbers marked * are at 2.5Mbps operation.

Jitter Filter

To build a network with transceivers that introduce big jitter like ones for optical fiber, the old HUB that has direction

control only may cause a transmission error because jitters on each HUB are added when several HUBs were

connected in serial. The TMC2005 fixes that problem with jitter filtering and wave shaping through the following

three steps.

1)

Input Sampling

The TMC2005 samples a data on a network by eight times clock of the network data.

2)

Jitter Filtering (DPLL)

The TMC2005 filters the jitter (± 100nS at 2.5Mbps) of network data sampled by 8X clock through the internal digital

PLL and stores the data into the buffer.

3)

Wave Shaping Output

The TMC2005 re-synchronizes and regenerates the network data at the same clock as the data rate.

The capability of the jitter filtering is shown below.

DATA RATE

CAPABILITY OF JITTER FILTERING

10Mbps

± 25nS

5Mbps

± 50nS

2.5Mbps

± 100nS

1.25Mbps

± 200nS

625Kbps

± 400nS

312.5Kbps

± 800nS

156.25Kbps

± 1.6uS