Inter-IC Encoding Technique

The 3B4B encoding method is used for communication between

ACS401s, thus ensuring that there is no DC component in the

signal. The encoding and decoding is transparent to the user.

Transmitter and Receiver Functions

Signals TxD and RxD in this specification refer to the set of signals

TxD(1:4) and RxD(1:4) respectively.

The TxD input data of the transmitting modem is time compressed

and encoded in 3B4B format. In the receiving modem, 3B4B

encoding ensures easy extraction of the bit-clock. The received

data is filtered, decoded, then stored in an output memory. The

memory provides time expansion, de-jittering and frequency

compensation. The data is finally directed to the RxD output pin.

Operational Modes

The ACS401 has four operational modes controlled by DP(1:4).

There are modes of operation to support PPLED and LASER

duplex devices on single fiber. In addition, LED/PIN and

LASER/PIN are supported on dual fiber.

Operational

No. of

DP4

DP3

DP2

DP1

Mode

Fibers

1. PPLED

Single

1

0

0

1

2. 4-pin Duplex

Single

1

0

0

0

3. LED & PIN

Dual

1

0

0

0

4. LASER & PIN Dual

1

0

0

0

N.B. All LASERs must be 4-pin LASERS. DP(1:4) combinations

not listed above are factory IC test modes, and should not be

selected by the user. Damage to the PPLED/Duplex & LED/PIN

components may result if these illegal modes are selected.

N.B. for all Operational modes 1, 2, 3, 4 SETB (Pin2) MUST be

tied High.

Mode 1 - LED single fiber mode

This is the operational mode for single fiber LED transmission, i.e.

the LED is a ‘ping-pong’ type (PPLED) used for both transmit and

receive. Connect PINP of ACS401 to GND and leave PINN floating.

Mode 2 4-pin Laser duplex single fiber mode

This is a single fiber mode where the Laser duplex device is

employed. This duplex device comprises a 4-pin Laser for

transmission and a PIN diode for reception in a single housing.

The Duplex devices are driven by the ACS401 in a half-duplex

manner, so potential cross-talk between the transmitter and

receiver is of no consequence.

It is necessary to use a small number of external components

(shown in the diagram) to support the power regulation technique.

The method for setting the Laser power is described in section

headed Control of LASER current .

VA+ is the +5V supply used to power the analogue of the ACS401.

AGND is analogue ground (pin 49 of the ACS401).The loop is

stable when the average current from the monitor pin equals the

current through variable resistor VR2. The voltage VC is stable at

this point and sets the laser drive current.

Mode 3 - LED/PIN dual fiber mode

This is a dual fiber mode where the LED is used for transmission

and a separate PIN diode is used for reception. This allows the

use of low cost standard LEDs.

Mode 4 4-Pin LASER/PIN dual fiber mode

This is a dual fiber mode where the 4-pin laser is used for

transmission and a separate PIN diode is used for reception.

It is necessary to use a small number of external components

(shown in the diagram) to support the power regulation technique.

The method for setting the Laser power is described in section

headed Control of LASER current .

VA+ is the +5V supply used to power the analogue of the ACS401.

AGND is analogue ground (pin 49 of the ACS401).The loop is

stable when the average current from the monitor pin equals the

current through variable resistor VR2. The voltage VC is stable at

this point and sets the laser drive current.

Single

Fiber

LED

LAP

LAN

LED single fiber mode

4-pin Laser duplex single fiber mode

with external Laser drive circuitry

Laser

PIN Diode

nF

nF

VC

LAN

VA+

Single

Fiber

VR2

50k

IN4001

PMOS

ZVP4424A

AGND

PINN

nF

PINP

Fiber

Fiber

LED/PIN dual fiber mode

LED

PIN diode

LAN

LAP

PINN

Laser

nF

VC

LAN

VA+

VR2

50k

IN4001

PMOS

ZVP4424A

AGND

PINN

Fiber

PIN Diode

Fiber

470 nF

4-pin Laser dual fiber mode with

external Laser drive circuitry

100 nF

Acapella Optical Modem IC

ACS401

128 kbps Optical Modem for Long Haul Transmission for Single/Dual Fiber applications

100

10

470