ACS405CS Semtech Corporation, ACS405CS Datasheet - Page 7

• Manufacturer Part Number: ACS405CS
• Description: Acapella Optical Modem ic
• Manufacturer: Semtech Corporation

If one of the 8-channel modes is selected by the
DR(4:1) inputs, then the ACS405CS must be
configured to expect Non-Return-to-Zero (NRZ)
transmission data using the POL1/2 settings. The
otherwise redundant TNEG and RNEG pins are then
available to carry 4 additional data channels. The
mapping of transmission pins for 8-channel NRZ
operation can be seen in Table 6.
There are three transmission modes available on
the ACS405CS selected by the DR(4:1) pins, these
are TX1, TX2 and HT modes.
In TX1 mode all of the data channels (TPN/RPN)
are related to the clocks TCLK1/RCLK1.
In TX2 mode there are two system clocks, half the
signals are aligned to the clock TCLK1/RCLK1 and
the remaining half are aligned to the clock TCLK2/
RCLK2.
For dual channel operation, channel 1 is aligned to
clock TCLK1/RCLK1 and channel 2 is aligned to
clock TCLK2/RCLK2.
For four channel operation, channels 1-2 are aligned
to clock TCLK1/RCLK1 and channels 3-4 are
aligned to clock TCLK2/RCLK2.
For eight channel operation, channels 1-4 are
aligned to clock TCLK1/RCLK1 and channels 5-8
are aligned to clock TCLK2/RCLK2.
Acapella recognises that a large number of
designers using the ACS405CS modem will be
employing the device for single channel T1 or single
channel E1 operation. In such cases, the available
bandwidth far exceeds the bandwidth required. To
Transmission Modes (TMode)
8-Channel Operation
TX1 mode
TX2 mode
HT mode
Channel Nos.
1
2
3
4
5
6
7
8
TPOS1
TNEG1
TPOS2
TNEG2
TPOS3
TNEG3
TPOS4
TNEG4
Input
Table 6
Output
RPOS1
RNEG1
RPOS2
RNEG2
RPOS3
RNEG3
RPOS4
RNEG4
7
exploit this otherwise redundant bandwidth, the
ACS405CS incorporates a High Tolerant (HT)
transmission mode. In HT mode, by virtue of on-
chip error-correction, the device is extremely tolerant
to data errors.
All modes in the ACS405CS generate regular unique
Synchronisation Words (SWs) in order to re-enforce
8B10B word boundary synchronisation. This
prevents single bit-errors causing an avalanche of
errors due to word boundary slippage, often referred
to as "error extension". In HT mode, SWs are
interleaved with the data-words such that alternate
words are SWs, ensuring that the effect of 8B10B
word boundary errors will not propagate beyond
one received data word.
In HT mode the single data channel is aligned to
the TCLK1/RCLK1 clock.
Some customers will require a modem capable of
supporting a longer distance than the standard
25km. To support this Acapella have implemented
a 50km mode for a single E1 or T1 channel. This
is achieved with an XTAL of value half of that used
for the standard E1 or T1 for 25km modes in
combination with the appropriate configuration of
the DR(4:1) pins.
NOTE: The designer should be aware that the use
of lower value crystal will proportionally increase
the end-to-end data delay through the system in
accordance with the formula found in section
headed, Data Delay (Latency). Therefore, for the
50km mode the latency will double compared to the
standard 25km modes.
A set of example configurations follow:
In order to select one E2 operation then set DR4 =
DR3 = DR2 = DR1 = 1, together with a crystal of
33.792MHz as shown in the Table 7.
In order to select one T2 channel then set DR4 =
DR3 = DR2 = 1; DR1 = 0, together with a crystal of
31.560MHz as shown in the Table 8.
Example Data Rate Selection
E2 Selection
T2 Selection
50km long haul mode
DR4
1
crystal = 33.792 MHz
DR3
1
DR2
1
DR1
1
4
XTAL
Divide
ACS405CS Issue 4.3 June 1999.
Table 7
(MHz)
TCLK
8.448