PEB 80900
Part II
’
IEC-T
’
: Functional Description
Data Sheet
111
2001-04-02
6.7.5.1
Access to MONITOR Channel in the IOM
-1 Mode
Detection of a valid command in the MONITOR channel will be acknowledged by setting
the E Bit to 0 in the frame containing the response. The E Bit will be reset to 1 for each
successive frame until a new valid command is detected.
If no relay driver pin is to be set or if no measurement value is requested, the MONITOR
channel (on DIN) must be held in a NOP condition. Then the NTC-T AM with IEC-T
functionality selected keeps the E Bit and the MONITOR channel at binary 1.
6.7.6
Readout Format of Echocanceller Coefficients
The NTC-T AM with IEC-T functionality selected contains an adaptive FIR filter with 48
coefficients. This filter emulates the line characteristics in order to cancel the echoed
data from the received signal and to regain the data from the far end. Because these
coefficients characterize the line, being a time discrete pulse response, it is of great
interest to read them out of the NTC-T AM with IEC-T functionality selected and evaluate
them to localize possible faults in the two wire loop.
The 16 most significant bits of the echocanceller coefficients can be read out via the
MONITOR channel, addressing the high byte and the low byte differently. To reduce
internal circuitry, these coefficients have a rather complicated format.
Figure 67
Transposition of EC Coefficient to MONITOR Channel
The low byte of a coefficient is to be accessed with an even numbered address in the
MONITOR channel, the high byte with an odd numbered address. The coefficient to
cancel the first echo is therefore accessible under addresses 04
H
and 05
H
; the last
coefficient is found in addresses 62
H
and 63
H
.
To avoid a carry from low to high, caused by continuous adaption between the readouts
of the low and the high byte, all coefficients are frozen for 125 μs, while one coefficient
byte is read. If each coefficient is read in two subsequent IOM
frames, no carry overflow
will occur and both bytes will fit together.
1
2
3
4
5
6
7
8
9
10
11
12
13
14 15
16
1
2
3
4
5
6
7
8
11 12
13
14
15
16
9
10
...
...
...
...
...
not readable
low byte
high byte
MSB
echo canceller coefficient
Format on MONITOR channel
LSB
Transposition_EC.emf