TI380C60A Datasheet, PDF (7/16 Pages) Texas Instruments – CMOS TOKEN-RING INTERFACE DEVICE

TI380C60A
CMOS TOKEN-RING INTERFACE DEVICE
SPWS033 – DECEMBER 1996
phantom driver and wire-fault detection (continued)
The watchdog timer is normally not allowed to expire because it is being reinitialized at least every 20 ms. If there
is a problem in the TI380C2x or its microcode, resulting in failure to toggle NSRT, the timer expires in a maximum
duration 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 them 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. 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 then 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 fault causes the wire-fault indicator
output, WFLT, to be driven low. The logic state of WFLT is high when the phantom drive is not active.
frequency acquisition and REDY
Unlike its predecessors, the TMS3805x family, the data-recovery PLL of the TI380C60A does not require
constant frequency monitoring; neither is it necessary to recenter its frequency by way of the FRAQ control line.
However, it is necessary to provide the interaction with the TI380C2x or other commprocessors that expect to
perform this frequency-management task.
When the TI380C2x asserts FRAQ, it initiates a reset of the clock-recovery PLL. The REDY signal is deasserted
for the duration of this action and reasserted low when it is complete (a maximum of 3 µs later). This low-going
transition of REDY is required by the TI380C2x following the setting of FRAQ high to indicate to the
commprocessor that any frequency error that it detected has been corrected. In fact, the TI380C60A never
requires FRAQ to be asserted after the PLL has been initialized. This interaction is provided purely for the benefit
of the TI380C2x. REDY is also de-asserted if no incoming transitions are detected by the rate error function.
When the TI380C60A is used as a standalone, it is necessary to initialize the device once following initial
power-up. This can be done in one of the three ways:
D A falling edge on FRAQ after power has stabilized
D A rising edge on PWRDN after power has stabilized. [However, note that the clock signals (PXTAL, RCLK,
OSC32) will not operate while PWRDN is low.]
D By toggling the S4 / 16 pin in either direction after power has stabilized
rate error (RATER) function
RATER provides an indication that incoming data transitions are present on the RCV+ / RCV– pair but the rate
of transitions is outside the range that would be expected for the ring speed selected by S4 / 16. RATER is not
asserted low if no incoming transitions are present. In wrap mode, the rate error function monitors the transitions
on the DRVR+/ DRVR– pair.
The rate error function interprets 16 or more transitions in a 1.5-µs period as valid 16-Mbps data. It interprets
15 or less transitions in a 1.5-µs period as 4-Mbps data. One transition or less in a 1.5-µs period is interpreted
as no incoming transitions—in which case, RATER and REDY are not asserted.
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