Maxim Integrated DS21Q55 - Manual (Page 48)

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8.2 T1 Transmit Transparency
The software-signaling insertion-enable registers, SSIE1–SSIE4, can be used to select signaling insertion
from the transmit-signaling registers, TS1–TS12, on a per-channel basis. Setting a bit in the SSIEx
register allows signaling data to be sourced from the signaling registers for that channel.
In transparent mode, bit 7 stuffing and/or robbed-bit signaling is prevented from overwriting the data in
the channels. If a DS0 is programmed to be clear, no robbed-bit signaling will be inserted nor will the
channel have bit 7 stuffing performed. However, in the D4 framing mode, bit 2 will be overwritten by a
zero when a yellow alarm is transmitted. Also, the user has the option to globally override the SSIEx
registers from determining which channels are to have bit 7 stuffing performed. If the T1TCR1.3 and
T1TCR2.0 bits are set to one, then all 24 T1 channels will have bit 7 stuffing performed on them,
regardless of how the SSIEx registers are programmed. In this manner, the SSIEx registers are only
affecting channels that are to have robbed-bit signaling inserted into them.
8.3 T1 Receive-Side Digital-Milliwatt Code Generation
Receive-side digital- milliwatt code generation involves using the receive digital- milliwatt registers
(T1RDMR1/2/3) to determine which of the 24 T1 channels of the T1 line going to the backplane should
be overwritten with a digital- milliwatt pattern. The digital- milliwatt code is an 8-byte repeating pattern
that represents a 1kHz sine wave (1E/0B/0B/1E/9E/8B/8B/9E). Each bit in the T1RDMRx registers
represents a particular channel. If a bit is set to a one, then the receive data in that channel will be
replaced with the digital- milliwatt code. If a bit is set to zero, no replacement occurs.