MT8926
The MT8926 PMAC can perform a payload loopback
by muxing the data received on the MT8976/77’s
DSTo to DSTi of the MT8976/77. This is controlled by
the MT8926 Loopback Control Word (CSTi1 channel
15, see Table 4).
Figure 6 illustrates the MT8926 payload loopback
connections. The ST-BUS transmit stream (normally
connected to DSTi of the MT8976/77) will connect to
DSTi0 of the MT8926. The DSTo stream of the
MT8976/77 will connect to DSTi1 of the MT8926 and
to the ST-BUS receive stream. DSTo of the MT8926
will connect to DSTi of the MT8976/77.
Line Loopback Codes
T1.403 defines SF mode line loopback activate and
deactivate codes. These codes are either a framed
or un-framed repeating bit sequence of 00001 for
activation or 001 for deactivation. The standard goes
on to say that these codes will persist for five
seconds or more before the loopback action is taken.
The MT8926 will generate line loopback activate and
de-activate codes. These functions are controlled by
the Loopback Control Word of CSTi1 channel 15
(see Table 4). The connections illustrated in Figure 6
must be implemented for this feature to function.
The MT8926 will also detect both framed and un-
framed line loopback activate and de-activate codes
even in the presence of a BER of 3 errors in 1000
bits. See the PMAC Miscellaneous Status Word,
Table 5, CSTo channel 7 bit 3 Line Loopback Enable
Detect (LLED) and bit 4 Line Loopback Disable
Detect (LLDD). The line loopback of T1.403 is
equivalent to the remote loopback function of the
MT8976/77 (see MT8976/77 data sheet, Master
Control Word 2). Therefore, the user will monitor the
LLED and LLDD bits to ensure that they persist for a
minimum of five seconds. Then the line loopback can
be either activated or de-activated using the
MT8976/77 Remote Loopback function.
PMAC Synchronization
The MT8926 has its own frame synchronization
mechanism, which uses the received signal (RxA
and RxB) and E8Ki to achieve SF and ESF frame
and superframe alignment. Further, the PMAC
monitors the SYN bit of the MT8976/77 (CSTo
channel 15 bit 0) and will not declare synchronization
until it is clear. When SYN is low and the MT8926
framer is properly aligned for two superframes, the
PMAC will declare synchronization by making the
Frame Error Count Validation bit (FECV) high. If this
criteria is not met, FECV will be low.
The PMAC will use the received F
T
and F
S
bits, F
T
bits only or FPS bits to acquire synchronization. The
receive F
T
or FPS bits are used to determine the out-
of-synchronization state (see Table 5).
Timer Outputs
The PMAC has two timer outputs, 1SEC pin and
CSTo TMR bit, which are derived from the 8 kHz ST-
BUS frame pulse F0i. These signals have been
implemented to provide the interface controller with a
timing reference for the transmission of T1.403/408
message-oriented performance data over the FDL.
See the Application section for an explanation of
T1.403/408 FDL Message Transfer.
The 1SEC output (pin 19) changes state once every
half second, and therefore, has a period of one
second. The relationship between the 1SEC output
and the frame pulse F0i is shown in Figure 16.
TMR is bit one of the PMAC Miscellaneous Status
Word (CSTo channel 7, see Table 5). It changes
state once per second on the rising edge of the
1SEC output, and thus, has a two second period.
Therefore, TMR will change state 62 C2i clock cycles
(or ST-BUS bit times) after the frame pulse that
immediately precedes the rising edge of the 1SEC
output.
Framing Error Event Counters
The MT8926 has two four bit counters, the Framing
Error Counter (FE) and the Severely Errored
Framing Event Counter (SE). The FE counter will be
incremented each time a single framing error in the
T1 signal is received. The SE counter will be
incremented by the reception of a T1 framing pattern
with an error rate that is greater than or equal to two
out of six bits. See Table 8 for errored frame event
counter details.
Frame #
1
2
3
4
5
6
7
8
9
10
11
12
F
T
1
F
S
0
0
0
1
1
0
1
1
1
0
0
Table 6. D3/D4 or SF Frame Pattern
Table 6 illustrates the Terminal Framing bits (F
T
) and
Signalling Framing bits (F
S
) of the SF or D3/D4
4-11
MITEL [ MITEL NETWORKS CORPORATION ]
