MT90220
from the TXCK and is independent from (not aligned
with) the RXSYNC or other TXSYNC signals.
•
•
Mode 7: Generic PCM interface for E1
applications, TXCK and TXSYNC inputs
Mode 8: ST-BUS interface for E1 applications,
TXCK and TXSYNC outputs
4.2 PCM System Interface Modes
T1 with ISDN services operation is available in PCM
Modes 1, 2, 5 and 6. Channel 24 is not used to carry
the ATM cells and is reserved for signaling. In this
case, 23 channels are used to carry the ATM cells
over the links and all the links in the IMA Group must
have the same option selected. The signaling
channel is always channel 24 in the T1 frame and its
position will vary with the PCM frame length.
There are 8 major modes of operation for the PCM
interface. The only difference between modes 1 to 4
and modes 5 to 8 is in the direction of the TXCK and
TXSYNC signals. In the PCM modes 5 to 8, the
direction is the opposite of what is defined in modes
1 to 4. The direction of the TXCK and TXSYNC
signals is defined by the bit 4 in the TX PCM Link
Control Register #2 and the PCM mode is defined
in the TX PCM Link Control Register #1
In PCM Mode 1 and 5, the clock can be either 1.544
or 2.048 MHz. If a 2.048 MHz clock is selected, the
mapping of the 24 channels can be either grouped
(the first 24 consecutive timeslots starting with the
Frame Pulse) or spaced (3 of every 4 timeslots,
starting with the second timeslot of the 32 timeslot
frame).
•
•
•
•
•
•
Mode 1: Generic PCM interface for T1
applications, TXCK and TXSYNC outputs
Mode 2: ST-BUS interface for T1 applications,
TXCK and TXSYNC inputs
Mode 3: Generic PCM interface for E1
applications, TXCK and TXSYNC outputs
Mode 4: ST-BUS interface for E1 applications,
TXCK and TXSYNC inputs
In PCM Modes 1, 3, 5 and 7, polarity of clock and
synchronization signals (TXCK, TX SYNC, RXCK
and RX SYNC) can be set as either positive or
negative. In the ST-BUS mode 2, 4, 6 and 8, the
Mode 5: Generic PCM interface for T1
applications, TXCK and TXSYNC inputs
Mode 6: ST-BUS interface for T1 applications,
TXCK and TXSYNC outputs
PCM Major Modes
TX PCM Control Registers 1 & 2
Reg 1 bit 6 Reg 1 bit 5 Reg 2 bit 4
Description
E1/T1
ST-BUS/
Generic
TXCK/TXSYNC
(Input/Output)
Output
Input
Mode 1
Mode 2
Mode 3
Mode 4
Mode 5
Mode 6
Mode 7
Mode 8
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
0
0
0
0
1
1
1
1
T1
T1
E1
E1
T1
T1
E1
E1
Generic
ST-BUS
Generic
ST-BUS
Generic
ST-BUS
Generic
ST-BUS
Output
Input
Input
Output
Input
Output
Table 6 - PCM Modes
.
PCM Mode
ST-BUS
GENERIC
Clock Frequency
Mapping
4.096 MHz
1.544MHz
2.048 MHz
a
b
c
Grouped
Spaced
Fixed
Fixed
Grouped
Spaced
Fixed
T1 ISDN
(23 channels)
Yes
Yes
Yes
--
--
Yes
Yes
Yes
--
Yes
--
T1 Clear Channel
(24 channels)
Yes
--
--
Yes
Yes
--
--
E1 (30 channels)
Yes
--
Yes
Table 7 - PCM Clock and Mapping Options
a. "Grouped" corresponds to the use of the first 23 or 24 channels (timeslots) from the 32 timeslots available in a frame.
b. "Spaced" corresponds to the use of 3 channels every 4 channels (timeslots) from the 32 timeslots available in a frame.
c. "Fixed" corresponds to the use of channels 1-15 and 17-31 leaving channels (timeslots) 0 and 16 not used.
24
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