ARCNET 5 Port HUB Controller

Datasheet

OPERATIONAL DESCRIPTIONDirection 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 (HYC9088A) of ARCNET normal mode. The nP1BAK is a pulse output pin for transceiver (HYC5000/4000/2000 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 RATECAPABILITY OF JITTER FILTERING

10Mbps

± 25nS

5Mbps

± 50nS

2.5Mbps

± 100nS

1.25Mbps

± 200nS

625Kbps

± 400nS

312.5Kbps

± 800nS

156.25Kbps

± 1.6uS

Revision 1.1 (07-24-07)

Page 10

SMSC TMC2005-JT

DATASHEET