ACS406CS Semtech Corporation, ACS406CS Datasheet - Page 14

• Manufacturer Part Number: ACS406CS
• Description: Two-chip Fiber Optic Modem Ics / ACS4060 100-pin Tqfp, ACS9010 44-pin TQFP
• Manufacturer: Semtech Corporation

component. The modem should be protected from
EMI/RFI sources in the standard ways.
The link budget is the difference between the power
coupled to the fiber via the transmit Laser/LED and
the power required to realise the minimum input-
amplifier current via the receive PIN/LED. The link
budget is normally specified in dB, and represents
the maximum attenuation allowed between
communicating Lasers/LEDs. The budget is utilised
in terms of the cable length, cable connectors and
splices. It usually includes an operating margin to
allow for degradation in LASER/LED performance.
The power coupled to the cable is a function of the
efficiency of the Laser/LED, the current applied to
the Laser/LED and the type of the fiber optic cable
employed.
The ACS4060 may be used as a controller and data
compression/decompression engine, which allows the
device to be used with in external circuitry for non-
fiber applications. Check with Acapella for details.
There are two conditions that will make LOSS go to
logic 1:
i) Loss of synchronisation - ping-pong windows
incorrectly aligned i.e DCD = 0.
ii) 64 received symbols break the 8B10B encoding
rules in a sequence of 256 symbols.
In order to return LOSS to the logic state 0 the
following criteria must be met.
i) The devices must be synchronised - ping-pong
windows correctly aligned i.e DCD = 1.
ii) There are no received symbols in a sequence of
256 symbols which break the 8B10B coding rules.
Although the ACS406CS is a full-duplex modem, at
the fiber level the device operates in a half-duplex
manner.
bidirectional transmission by alternating the direction
of data flow. This means that data must be stored
until the link is configured in the appropriate direction.
Storage inevitably leads to delay or latency. Acapella
has designed the ACS406CS to minimise latency by
very rapidly switching direction at the fiber level,
minimising the need to store data.
Link Budgets
Digital Mode
LOSS ( Loss Of Synchronisation)
Data Delay (Latency)
Typically, half-duplex systems allow
14
The latency through the system applies to the main
data TPOS/TNEG channels and the support channels
TmD/RmD and is a function of the XTAL frequency.
For a given implementation, the latency has three
components:
1) A Constant Delay (CD) set by the machine cycle
and system clock at a frequency of 32.768MHz then
CD = 0.7ms.
2) A Transmission Rate Delay (TRD) which is equal
to 125 bit periods worst case (de-jittering buffer and
internal registers).
3) Fiber length Dependent Delay (FDD) which is equal
to 2 * (fiber delay).
The Latency formula is:
Latency =
(0.7 * 10
* fiber delay)
where the
Example Latency calculation
Latency calculation:
Fiber length
XTAL
DR mode
Latency = ((0.7 * 10
10
-9
) + ( 2* 50 * 10
-3
* 32.768 * 10
fiber delay is typically 5us per km.
= 10km ( ~ 50us)
= 33.792MHz
= 4 * 2.048Mbps ( transmit bit period = 488ns)
-3
-6
* 32.768 * 10
)) = 0.84ms
-6
/XTAL) + (125 * transmit bit period) + (2
-6
/33.792 * 10
ACS406CS Issue 1.6 January 1999.
-6
) + (125 * 488*