XRT74L74 Datasheet, PDF (317/498 Pages) Exar Corporation – 4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

PRELIMINARY
XRT74L74
4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.1.1
TABLE 64: DESCRIPTION OF METHOD 1 TRANSMIT OVERHEAD INPUT INTERFACE SIGNALS
NAME
TxOHIns
TxOH
TxOHClk
TxOHFrame
TYPE
Input
Input
Output
Output
DESCRIPTION
Transmit Overhead Data Insert Enable input pin.
Asserting this input signal (e.g., setting it "High”) enables the Transmit Overhead Data Input Inter-
face to accept overhead data from the Terminal Equipment. In other words, while this input pin is
"High", the Transmit Overhead Data Input Interface will sample the data at the TxOH input pin, on
the falling edge of the TxOHClk output signal.
Conversely, setting this pin "Low" configures the Transmit Overhead Data Input Interface to NOT
sample (e.g., ignore) the data at the TxOH input pin, on the falling edge of the TxOHClk output
signal.
NOTE: If the Terminal Equipment attempts to insert an overhead bit that cannot be accepted by
the Transmit Overhead Data Input Interface (e.g., if the Terminal Equipment asserts the TxOHIns
signal, at a time when one of these non-insertable overhead bits are being processed), that par-
ticular insertion effort will be ignored.
Transmit Overhead Data Input pin:
The Transmit Overhead Data Input Interface accepts the overhead data via this input
pin, and inserts into the overhead bit position within the very next outbound E3 frame. If
the TxOHIns pin is pulled "High", the Transmit Overhead Data Input Interface will sam-
ple the data at this input pin (TxOH), on the falling edge of the TxOHClk output pin.
Conversely, if the TxOHIns pin is pulled "Low", then the Transmit Overhead Data Input
Interface will NOT sample the data at this input pin (TxOH). Consequently, this data will
be ignored.
Transmit Overhead Input Interface Clock Output signal:
This output signal serves two purposes:
1. The Transmit Overhead Data Input Interface will provide a rising clock edge on this signal, one
bit-period prior to the instant that the Transmit Overhead Data Input Interface is processing an
overhead bit.
2. The Transmit Overhead Data Input Interface will sample the data at the TxOH input, on the fall-
ing edge of this clock signal (provided that the TxOHIns input pin is "High").
NOTE: The Transmit Overhead Data Input Interface will supply a clock edge for all overhead bits
within the DS3 frame (via the TxOHClk output signal). This includes those overhead bits that the
Transmit Overhead Data Input Interface will not accept from the Terminal Equipment.
Transmit Overhead Input Interface Frame Boundary Indicator Output:
This output signal pulses "High" when the XRT74L74 is processing the last bit within a
given E3 frame.
The purpose of this output signal is to alert the Terminal Equipment that the Transmit
Overhead Data Input Interface block is about to begin processing the overhead bits for a
new E3 frame.
Interfacing the Transmit Overhead Data Input In-
terface to the Terminal Equipment.
Figure 120 illustrates how one should interface the
Transmit Overhead Data Input Interface to the Termi-
nal Equipment, when using Method 1.
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