TI380C30A Datasheet, PDF (22/81 Pages) Texas Instruments – INTEGRATED TOKEN-RING COMMPROCESSOR AND PHYSICAL-LAYER INTERFACE

TI380C30A
INTEGRATED TOKEN-RING COMMPROCESSOR
AND PHYSICAL-LAYER INTERFACE
SPWS034 – MARCH 1998
receiver-clock recovery (continued)
PLL1 represents the algorithm to recover data from the incoming stream detected by the receiver. It has a
relatively high bandwidth to provide good jitter tolerance. Data and embedded-clock-phase information are fed
as digital values to PLL2, which generates the extracted clock (RCLK) for the commprocessor. The recovered
data is sent to the commprocessor as the RCVR signal in synchronization with RCLK. In addition to sampling
the RCVR signal, the commprocessor uses RCLK to retransmit data in most cases. The lower bandwidth of
PLL2 greatly reduces the rate of accumulation of data-correlated phase jitter in a token-ring network and
provides very good accumulated-phase-slope (APS) characteristics. In addition to RCLK, the token-ring
reference clock (PXTAL) and a fixed-frequency 32-MHz clock (OSC32) are also synthesized from the 8-MHz
crystal reference.
line driver and wrap function
The line-drive function of the TI380C30A is performed by XMT+/XMT–. Unlike the TMS38054, these pins are
low-impedance outputs and require external-series resistance to provide line termination. These pins provide
buffering of the differential signal from the PH on DRVR+/DRVR– with action to control skew and asymmetry,
and with no retiming in the transmit path.
The wrap function is designed to provide a signal path for system self-test diagnostics. When the PH drives
WRAP low, the receiver inputs are ignored and the transmit signal is fed to the receiver input circuitry by way
of a multiplexer. In the internal wrap mode, WRAP can be checked by observing the signal amplitude at the
equalization pins, EQ+ and EQ–. Equalization is active at this signal level, although the signal does not exhibit
the high-frequency attenuation effects for which equalization is intended to compensate. During wrap mode,
both XMT+/XMT– are driven to a low state to prevent any dc current from flowing into the isolation transformer.
phantom driver and wire-fault detection
The phantom-drive circuit under control of NSRT generates a dc voltage on both of the phantom-drive outputs,
PHOUTA and PHOUTB. To maintain the phantom drive, NSRT is toggled by the TI380C30A at least once every
20 ms. A watchdog timer is included in the TI380C30A to remove the phantom drive if NSRT does not have the
required transitions.
The watchdog timer normally is not allowed to expire because it is being reinitialized at least every 20 ms. If there
is a problem in the TI380C30A or its microcode, resulting in failure to toggle NSRT, the timer expires in a
maximum of 22 ms. If this happens, the phantom drive is deasserted and remains so until the next falling edge
of NSRT. The watchdog timer requires no external-timing components. When the phantom drive is deasserted,
the phantom-drive lines are actively pulled low, reaching a level of 1 V or less within 50 ms.
The dc voltage from PHOUTA and PHOUTB is superimposed on the transmit-signal pair to the trunk-coupling
unit (TCU) to request that the station be inserted into the ring. This is achieved by connecting the transmit-signal
pair to the center of the secondary winding of the transmit-isolation transformer. Since PHOUTA and PHOUTB
are connected to the media side of the isolation transformer, they require extensive protection against line
surges. A capacitor is connected between the two phantom lines to provide an ac path for the transmit signal,
while PHOUTA and PHOUTB independently drive the dc voltage on each of the transmit lines, allowing for
independent wire-fault detection on each.
The phantom voltage is detected by the TCU, causing the external wrap path from the transmitter outputs back
to the receiver inputs to be broken and the ring to be broken. A signal connection is established from the ring
to the receiver inputs and from the transmitter outputs to the ring. The return current from the dc-phantom
voltage on the transmit pair is returned to the station by way of the receive pair. This provides some measure
of wire-fault detection on the receive lines. The phantom-drive outputs are current limited to prevent damage
if short-circuited. They detect either an abnormally high or an abnormally low load current at either output
corresponding to a short or an open circuit in the ring or TCU wiring. Either type of fault results in the wire-fault
indicator output (WFLT) being driven low. The logic state of WFLT is high when the phantom drive is not active.
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