Rainbow Electronics DS26504 - Manual (Page 64)

DS26504 T1/E1/J1/64KCC BITS Element
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12. E1 SYNCHRONIZATION STATUS MESSAGE
The DS26504 provides access to both the transmit and receive Sa/Si bits. In E1, the Sa bits are used to
transmit and receive the SSM. The primary method to access the Sa (and Si) bits is based on CRC4
multiframe access. An alternate method is based on double-frame access. The DS26504 provides an
interrupt on a change of state for the Sa-bit-based messages.
Table 12-1. E1 SSM Messages
QUALITY
LEVEL
DESCRIPTION
Sa BIT
MESSAGE
0 Quality unknown (existing sync network) 0000
1 Reserved 0001
2 Rec. G.811 (Traceable to PRS) 0010
3 Reserved 0011
4 SSU-A (Traceable to SSU type A, see G.812) 0100
5 Reserved 0101
6 Reserved 0110
7 Reserved 0111
8 SSU-B (Traceable to SSU type B, see G.812) 1000
9 Reserved 1001
10 Reserved 1010
11 Synchronous Equipment Timing Source 1011
12 Reserved 1100
13 Reserved 1101
14 Reserved 1110
15 Do not use for synchronization 1111
In E1 operation, SSMs are transmitted using one of the Sa bits—Sa4, Sa5, Sa6, Sa7, or Sa8. The SSM is
transmitted MSB first in the first frame of the multiframe. Each multiframe will contain two SSMs, one in
each sub-multiframe. An SSM is declared valid when the message in three sub-multiframes are alike.
12.1 Sa/Si Bit Access Based on CRC4 Multiframe
On the receive side, there is a set of eight registers (RSiAF, RSiNAF, RRA, RSa4 to RSa8) that report the
Si and Sa bits as they are received. These registers are updated on CRC4 multiframes. A bit in Status
Register 4 (SR4.1) indicates the multiframe boundary. The host can use the SR4.1 bit to know when to
read these registers. The user has 2ms to retrieve the data before it is lost. The MSB of each register is the
first received. See the following register descriptions for more details.
On the transmit side, there is also a set of eight registers (TSiAF, TSiNAF, TRA, TSa4 to TSa8) that, via
the transmit Sa bit control register (TSaCR), can be programmed to insert both Si and Sa data. Data is
sampled from these registers with the setting of the transmit multiframe bit in status register 2 (SR4.4).
The host can use the SR4.4 bit to know when to update these registers. It has 2ms to update the data or
else the old data will be retransmitted. The MSB of each register is the first bit transmitted. See the
following register descriptions for details.