ACS103 Issue 2.03 May 1996.

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synchronisation the machine cycle reverts

automatically to normal.

Note that only one device can be configured in active

lock at any one time, and thus the DM pins must not

be permanently wired High on both devices in a

production system.

Active lock mode is usually

invoked on power-up.

One common way of

temporarily invoking active lock is to adopt the

standard RC time-constant method. This is achieved

on the ACS103 by connecting DM1, DM2 and DM3

together, and attaching that node to an RC

arrangement, i.e. with the capacitor to 5 V and the

resistor to ground. This creates a 5 V

à 0 V ramp on

power-up. The RC time-constant should be Ca. 1

second.

Random Lock

This is a new mode of operation (over the ACS100),

both ends of a link can be permanently configured in

this mode (i.e. with hard-wired DM1-DM3 pins),

which will achieve lock in typically 1 second, and

worst case 5 seconds.

Like active lock, random lock will operate even when

both ends of the link are driven by identical clock

frequencies (0 ppm).

Random lock mode is

compatible with drift lock available on the ACS100.

Mixing Lock modes

It is possible to mix all combinations of locking

modes once the modems are locked, however, prior

to synchronisation two modems configured in active

lock will not operate. Normally, random lock will be

the preferred mode.

The effect of mixing locking

modes on locking speed is tabulated below:

Device A

Device B

Locking Speed

Mode

Mode

Drift

Drift

Drift

Drift

Active

Active

Drift

Random

Random

Active

Active

Not allowed

Active

Random

Random

Random

Random

Random

LIN (Lock Indicator)

LIN goes High when synchronisation or 'lock' is

achieved. Lock is normally an invert of DCDB. But

unlike DCDB is not affected by the status of RTS and

DTR, or the selected diagnostic mode.

ERD (Error Detector)

This signal can be used to give an indication of the

quality of the optical link. Even when a DC signal is

applied to the TxD and TxCL inputs, the ACS103

transmits approximately

256 kbps over the link in

each direction. This control data is used to maintain

the timing and the relative positioning of transmit and

receive windows. The transmit data and the control

data is constantly monitored to make sure it is

compatible with the 3B4B format.

If an error is

detected then ERD will go High and will remain High

until ERD is initialised. ERD may be initialised by

applying reset DM1-DM3 or PORB, or by removing

the fiber-optic cable from one side of the link thereby

forcing the device temporarily out of lock. ERD is

only an indication and is not a substitute for BER

tests.

LED Considerations

Since LEDs from different suppliers may emit

different wavelengths, it is recommended that the

LEDs in a communicating pair of modems are

obtained from the same supplier.

The emission

spectrum of an LED is a function of temperature, so a

temperature difference between the ends of a link

reduces the responsivity of the receiving diode. This

results in a reduction in the link budget. Information

is given in the LED suppliers’ data sheets.

The

following manufacturers have LEDs that have been

successfully tested with the ACS103 and Acapella

will be glad to assist with contact names and

addresses on request.

Suppliers:

ABB-Hafo

(e.g. 1A-212-connector)

Acapella

(e.g. A-connector)

GCA

(e.g. 1A-212-connector)

Honeywell

(e.g. HFE4214-013, HFE4404-013)

Optek

(e.g. OPF372, OPF322)

Most suppliers support the standard range of fiber

connectors, e.g. ST, SMA and FC.

Power Supply Decoupling

The ACS103 contains a highly sensitive amplifier,

capable of responding to extremely low current

levels.

To exploit this sensitivity it is important to

reduce external noise to a low level compared to the

input signal from the LED. The modem should have

an independent power trace to the point where power

enters the board.

Figure 3. shows the recommended power supply

decoupling. The LED should be sited very close to

the LDN and LDP pins. A generous ground plane

should be provided, especially around the sensitive

LDN and LDP pins. The modem should be protected

from EMI/RFI sources in the standard ways.

Link Budgets

The link budget is the difference between the power

coupled to the fiber via the transmit LED and the

power required to realise a current of 650 nA

(minimum amplifier sensitivity) via the receive

LED.The link budget is normally specified in dB or

dBm, and represents the maximum attenuation

allowed between communicating 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 LED

performance.