Example: To generate a 38.4 kbps dual channel.

Select a 64 kbps dual channel using the data rate selection pins

DR4/3/2/1 = 1/1/0/0, and use a XTAL frequency of:

XTAL = (38.4 / 64) * 9.216 MHz = 5.5296 MHz.

Other ‘non-standard’ transmission frequencies may be generated

in the same way as long as the 1 - 12 MHz XTAL oscillator range

is observed. A wider range of external clock frequencies may also

be permissible - please check with Acapella.

Cint Capacitor - CNT

For a XTAL frequency range of 5 - 12 MHz the ACS401 requires

a ceramic capacitor of value 22 nF - 33 nF +/- 20 % between pin

CNT and GND. At frequencies lower than 5 MHz a capacitor of

value 68 nF - 100 nF is recommended. It is essential that the CNT

capacitor is placed very close to the ACS401.

DCDB

The Data Carrier Detect (DCDB) signal will go Low when the

modems are locked and ready for data transmission. Prior to lock

(DCDB = High), the data channels outputs RxD(1:4) are forced

Low with the control lines XO(1:2) forced High.

Data Rate Selection

The following data rates apply to TxD1, TxD2, TxD3 and TxD4 and

are based on the use of a 9.216 MHz XTALor clock.

‘Standard’ mode, DR4 = 1:

No. of

DR4

DR3

DR2

DR1

Data Rate

Channels

1

0

0

0

128 kbps

Single

1

0

0

1

64 kbps

Single

1

0

1

0

32 kbps

Single

1

0

1

1

16 kbps

Single

1

1

0

0

64 kbps

Dual

1

1

0

1

32 kbps

Dual

1

1

1

0

32 kbps

Four

1

1

1

1

16 kbps

Four

‘Double’mode, DR4 = 0:

No. of

DR4

DR3

DR2

DR1

Data Rate

Channels

0

0

0

1

64 kbps

Single

0

0

1

0

32 kbps

Single

0

0

1

1

16 kbps

Single

0

1

0

1

32 kbps

Dual

0

0

0

0

16 kbps

Dual

0

1

1

1

16 kbps

Four

In ‘standard’ mode internal timing accommodates fiber length

delays of up to 95 km, link budget permitting. In ‘double’mode the

internal timing accommodates fiber length delays of up to 190 km,

link budget permitting. ‘Double’ mode essentially halves the

frequency of the internal clock and therefore doubles the period of

the internal machine cycle, this also has the effect of doubling the

duration of LED/LASER transmit pulses, which in turn is likely to

lead to improved link budgets, particularly where LEDs or LASERs

of higher than recommended capacitance are employed.

Bandwidth * Channel Product

The ACS401 has a Bandwidth Channel Product (BCP) of 128 kHz

in ‘standard’ mode and 64 kHz in ‘double’ mode using the

recommended XTAL frequency of 9.216 MHz. The BCP is

proportional to the XTAL frequency and is specified as:

BCP(kHz) = 128 * XTAL / ( 9.216 *10

XTAL = frequency of XTALoscillator. n = 1 for ‘standard’mode; n

= 2 for ‘double’ mode.

The given maximum bandwidth may be shared by up to 4

channels as shown in the following tables for both ‘standard’ and

‘double’ modes and a XTAL frequency of 9.216 MHz.

‘Standard’ mode - XTAL = 9.216 MHz

Control of LASER current

The LASER output current must be set for each individual device

in accordance with the manufacturer’s recommendations. The

maximum output current to the Laser is controlled by the resistor

Rtrc connected between ground and TRC. The minimum value set

on Rtrc to avoid damage to the ACS401 is 800

Ω.

LASER(current max) = 100/Rtrc

tolerance +/- 25 %.

Whilst TRC sets the maximum current ( to prevent damage to the

laser during the adjustment procedure), the actual current to the

Laser is determined by the external component VR2. The

adjustment procedure is described in the following section.

Adjustment Procedure

Select the appropriate mode using the pins DP(1:4) (see section

headed Operational Modes). Choose a value for the resistor Rtrc

that delivers sufficient current to correctly drive the laser at the

desired power, with sufficient power margin to compensate for

temperature/voltage changes and potential laser degradation. In

many applications the value of Rtrc will be set to the maximum

current using a resistor value of 800

Ω.

The output power from the laser may be measured directly using

an optical-power meter that is capable of detecting peak optical-

power. If an average optical-power meter is employed then a

correction factor of 16 must be used to obtain the peak value .

LASER(peak power) = Laser(average power) * 16.

The external component VR2 should have a range of 0 to 50 K

Ω.

As VR2 is decreased, output power will first be detected when the

laser begins to lase. VR2 should be reduced further until the

desired output power is achieved (within the limits of the

manufacturer’s specification).

LED current control

The LED transmit current is less critical though it is important not

to exceed the LED manufacturer’s recommendation for maximum

current. The current is controlled by a resistor Rtrc connected

between TRC and ground. The lower the value Rtrc, the greater

the current. The lower limit for Rtrc is 800

Ω while a practical

maximum is 40 k

Ω.

The LED current is inversely proportional to Rtrc while Rtrc > 800

Ω:

LED(current) = 100/Rtrc

tolerance +/- 25 %.

PORB

The Power-On Reset or PORB pin resets the device if forced Low

for 100 ms or more. In normal operation PORB should be held

High. Although the PORB pin has been included, e.g. for factory

test, the modem has been designed to power up correctly without

the aid of PORB.

PORB has a special function when used in conjunction with

memory lock (see section headed Diagnostic Modes).

Crystal Clock

Normally, a parallel resonant crystal will be connected between

the pins XLI and XLO with the appropriate padding capacitors.

Alternatively, it is possible to drive XLI directly by an external

clock.

The clock frequency for the purpose of this specification will be

known as XTAL frequency.

The operational range for XTAL

frequency is 1 - 12 MHz, though communicating ACS401s must

be clocked at the same nominal frequency.

The ACS401 has

been designed to operate with a XTAL tolerance of 100 ppm

giving a relative tolerance of 200 ppm between communicating

modems.

The recommended frequency of 9.216 MHz, results in the

standard range of synchronous communication frequencies

tabulated in the section headed Data Rate Selection. Non-

standard frequencies may be generated by using the appropriate

value XTALor external clock.