AM7968 AMD [Advanced Micro Devices], AM7968 Datasheet - Page 99

• Manufacturer Part Number: AM7968
• Description: TAXIchip Integrated Circuits(Transparent Asynchronous Xmitter-Receiver Interface)
• Manufacturer: AMD [Advanced Micro Devices]

TAXI TIPs
TAXI Technical Information Publication #89-01
Subject: Receiver Response to Loss of Input Signal
Question:
It is desired that the TAXI Receiver outputs be predictable and stable during conditions
when the TAXI Transmitter may cease transmitting (power-off) or is disconnected. How
can a system designer predict the TAXI Receiver outputs or use the TAXI Receiver in a
system where the TAXI Receiver must appear Inactive under these conditions? This
applies to both Coaxial and Fiber-Optic Systems.
Answer:
The key to this problem is interpreting the loss of incoming Tx data as a Quiet-Line-
State and either flagging the system accordingly or gating the TAXI Receiver outputs
with an inactive flag.
In a Coaxial coupled system the loss of incoming signal drive will cause the TAXI
Receiver inputs to rest at the input termination bias voltages. The differential serial
inputs will normally be at equal potential. To properly interpret a loss of signal as a
constant quiet state the input termination bias voltages must be altered slightly from
each other. To allow reliable interpretation of the offset as a constant logic state, the
offset voltage should typically be set to about 50 mV.
The TAXI Receiver will interpret the Quiet-Line-State differently depending upon the
operation mode (8-bit, 9-bit, or 10-bit). In 8-bit mode the Receiver will generate continu-
ous CSTRB’s with the Command outputs at F Hex, or all HIGH. In the 9-bit and 10-bit
modes there is no defined interpretation of an incoming quiet data stream. This will
cause the TAXI Receiver to generate continuous CSTRB’s and the Violation output will
be continuously HIGH. A one-shot may be used to determine Violation=HIGH duration
and then generate an inactive flag, or the system may interpret the Violation output
directly as an inactive or invalid condition flag and halt data-dependent system opera-
tions during any byte with Violation=HIGH.
In a Fiber-Optic coupled system two methods may be used, depending on the Optical
Receiver construction. If the Optical Receiver has a Carrier-Detect output this signal
may be used to flag an inactive state. If there is no Carrier-Detect , one may be gener-
ated using an ECL one shot arrangement which will detect loss of edges after a
predetermined period of say, for example, 80 bit times. The actual time may vary
depending on the desired response to loss of data.
In either condition, if the Receiver recovered Byte Clock (CLK) is used as a system
clock, then the system must be able to tolerate a shift in the CLK frequency of typically
about +/-3% to allow for Rx-PLL offset and drift during this period. (If the Optical
Receiver begins oscillation when the fiber is dark, the TAXI Receiver PLL may attempt
to track the oscillation resulting in an indeterminate Rx recovered clock frequency.
Optical Receiver dark response thus becomes a possible system concern.)
TAXIchip Integrated Circuits Technical Manual
95