2.0 Functional Description
CN8330
2.3 Transmitter Operation
DS3/E3 Framer with 52 Mbps HDLC Controller
In C-bit parity mode with internal sourcing of the C-bits, FEBE alarms are
generated automatically in the transmitter when the receiver detects either a frame
bit error or a C-bit parity error in an M-frame. The 3-bit FEBE pattern that is
transmitted is contained in the FEBE Pattern Bit Field [FEBEC[3:1];CR04.2:0] of
the Feature Control Register. Bit 2 is placed in the C1 position of subframe 4
during the alarm condition, bit 1 is placed in the C2 position, and bit 0 is placed in
the C3 position. When no alarm condition is present, the FEBE channel contains
all ones. Therefore, to prevent disabling proper FEBE operation, the FEBE field
should be written to any combination other than 111.
An unframed, all-ones signal can be transmitted by setting both TxAlm0 and
TxAlm1 to a 1 and ExtOvh cleared to a 0.
In E3 mode, the AIS (unframed all-ones) is enabled by setting TxAlm0 high.
The yellow alarm is enabled by setting TxAlm1 high. This causes the transmitted
A-bit to be set to a one.
2.3.6 Terminal Data Link Transmitter
The terminal data link transmitter consists of the three C-bits in subframe 5 of the
M-frame in DS3 mode or the N-bit in E3 mode and uses the LAPD protocol.
When the Send Message bit [TxMsg;CR01.0] of the Terminal Data Link Control
Register [CR01;0x01] is low, the data link transmitter is continuously sending idle
flags (01111110). When high, message transmission is initiated. Transmission of
the FCS bytes is controlled by the Send Frame Check Sequence bit
[TxFCS;CR01.1]. If a nonstandard FCS is required, it may be loaded into the
message buffer and transmitted as part of the message. A message in progress
may be abandoned by setting the Abort Message bit [TxAbort;CR01.2]. This will
cause an abort flag (11111110) to be transmitted, followed by idle flags, until the
Send Message bit is reactivated. Two consecutive messages may share ending and
beginning idle flags. The transmitter can be disabled by setting the Disable
Terminal Data Link Transmission bit [DisTxTDL;CR01.6] to a one. This causes
all ones to be sent in the data link bit positions.
The terminal data link transmitter is under control of the C-Bit Parity
Mode/Enable DLINT/SOURCELB (CBitP/DL;CR00.0), E3 Framing Mode
[E3Frm;CR00.1], External C-Bit Insert, and Transmit Alarm Control bits in the
Mode Control Register. In the Terminal Data Link Control Register, the
transmitter is under the control of the TxMsg, TxFCS, TxAbort, Send Byte
[TxByte[2:0];CR01.5:3], and DisTxTDL bits. An interrupt for use with data link
operations is available on the DLINT/SOURCELB output pin and status bits for
determining the interrupt source are located in the Data Link Interrupt Status
Register [SR02;0x12].
The framer mode must be set to either C-bit parity mode or E3 mode for
terminal data link transmission to take place. In C-bit parity mode, the three
C-bits in subframe 5 of the M-frame will be used for transmission. In E3 mode,
the N-bit will be used for transmission. When E3 mode is selected, the CBitP/DL
bit must also be set to enable interrupts on the DLINT/SOURCELB pin. If neither
mode is set, terminal data link transmission will not take place. If the ExtCBit bit
is set in E3 mode, terminal data link transmission will be disabled. In C-bit parity
mode, ExtCBit has no effect on terminal data link transmission. Setting the
Transmit Alarm Control [1,0] bits for transmission of AIS or all ones in either
C-bit parity mode or E3 mode will disable transmission of the terminal data link.
Transmission of yellow alarms or idle codes has no effect on terminal data link
transmission.
2-12
Conexant
100441E
CONEXANT [ CONEXANT SYSTEMS, INC ]
