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Advanced Communications

ACS104A Data Sheet

Dual Fiber Modem for Asynchronous Data Rates from DC to 160kbps

current. The current is controlled by a resistance Rtrc connected

between TRC and GND. The lower the value of Rtrc the greater

the current.

The lower limit for Rtrc is 800

Ω while a practical

maximum is 40k

Ω.

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

Ω.

LED current = (100 / Rtrc) +/- 25 %

Data-Rate Selection

The ACS104A benefits from data compression circuitry which

reduces power consumption and improves the BER (Bit Error

Rate).

The compression technique employed, demands a

minimum TxD data-bit time of 10 sample-clocks. This defines the

maximum data rate:

Maximum data rate = sample-clock/10

However, an allowance must be made for any variation in the TxD

data-bit period to accommodate frequency variation and jitter.

Hence the maximum data rates specified in the following are

decreased by 10% to include a sufficient safety margin.

The ACS104A includes an input pulse shaper which ensures that

the system is very tolerant to jitter, and helps achieve a maximum

data-rate close to the theoretical maximum of sample-clock/10

(bps). The pulse shaper will expand data pulses of less than 10

clock-samples to meet the compression criteria. This is performed

on up to three consecutive data-bits which fail to meet the

minimum pulse width criteria.

DR3

DR2

DR1

XTAL

Sample

Max TxD

Clock

Clock

Data Rate

0

1

1

10MHz

XTAL/160

5.6kbps

1

0

0

10MHz

XTAL/80

11kbps

1

0

1

10MHz

XTAL/40

22kbps

1

1

0

10MHz

XTAL/20

45kbps

1

1

1

10MHz

XTAL/15

60kbps

1

1

1

20MHz

XTAL/15

120kbps

1

1

1

27MHz

XTAL/15

162kbps

Table 1. TxD Data-Rate Selection

Table 1. shows the maximum TxD data rate, which includes a 10%

tolerance margin, when using various frequency crystals, other

sample-clock frequencies may be generated by using the

appropriate value XTAL in combination with the divide constant

selected by DR(1:3) namely 15, 20, 40, 80 or 160. The advantage

of using a slower crystal and a lower sample clock is the reduced

power consumption of the device.

DR3 is internally held high in the PLCC28 package.

RS-232 Handshake Signals / Low Frequency Data Channels

Three additional low frequency data channels are provided on the

ACS104A which are often used for the RS-232 handshake signals.

The RS-232 handshake signals comprise the set RTS, CTS, DTR

and DSR. These are treated as pass through data channels rather

than using local handshaking. Hence the status of inputs RTS and

DTR appear at the far-end outputs CTS and DSR respectively. An

extra data channel has also been provided in the TQFP44

package, which may be used for sending the RS232 Ring Indicator

signal, for example. The input and output lines are RII and RIO

respectively.

The transmission method employed on the ACS104A has been

designed to give low skew (1 - 2 data-bits) on the main RTS, CTS,

DTR and DSR hanshake signals relative to the main TxD/RxD data

channel, while maintaining low power consumption.

The handshake signals are updated by two stimuli:

i.

An internal interval timer at a frequency proportional to the XTAL; at

10.0MHz this is approximately 1.6ms.

ii.

Changes detected on RTS and DTR.

The maximum bandwidth for the handshake signals may be

Transmitter and Receiver Functions

This device offers one high speed and three low speed full duplex

channels to the user in a completely transparent way, appearing as

4 full duplex channels.

Data from the TxD and low frequency channels is time

compressed in an internal FIFO and sent over the fiber link in a

burst within a predefined window. The two devices at each end of

the link automatically synchronise with each other such that the

transmit and receive windows are interleaved.

This technique

offers superior performance compared to continuous receive and

transmit systems, since the transmitted data does not cause noise

injection during the receiving time.

The TxD input data of the

transmitting modem is also data compressed. The 3B4B encoding

method is used for communication between ACS104As, thus

ensuring that there is no DC component in the signal.

The

encoding and decoding is transparent to the user.

In the receiving modem, 3B4B encoding ensures easy extraction of

the bit-clock.

The received data is filtered, decoded, and then

stored in the output memory.

The memory provides time

expansion, de-jittering and frequency compensation functions. The

data is then decompressed and directed to the RxD output pin,

appearing after a minimal delay, in the same format as that

presented at the TxD pin at the far end.

Operational Modes

The ACS104A is compatible with the ACS104 but offers over twice

the max data rate.

The following sections detail the operating

modes.

Additional modes are also described for new ways of

interfacing the device with external PIN / amplifier modules. The

setups refer to the pin settings for the TQFP44 package.

The

PLCC28 pin settings will be the same with the unavailable pins

being internally pulled high.

Mode 1 - Dual Fiber LED/PIN mode

Setup : DP4=1, DP3=1, DP2=1, DP1=1

This is a twin-fiber mode where the LED is used for transmission

and a separate PIN Diode is used for reception. An example circuit

diagram showing the necessary connections is shown in figures 3

and 4.

Mode 2 - Preamp Voltage Input & LED Drive

Setup : DP4=0, DP3=1, DP2=0, DP1=0, NSB=0 (TQFP44 only)

This is a mode for use with external amplifier and PIN modules. An

LED is used for transmission and connected as normal with its

anode to LAP and cathode to LAN. The differential voltage from

an external PIN/TIA module is connected to PINN and PINP via

100pF capacitors to provide DC isolation. The signals should be

connected such that PINP is connected to the TIA output that goes

high when light is received. A single input can also be applied from

a single ended PIN/TIA by feeding the input to PINP only, PINN is

left floating. This mode uses the new NSB pin, in all other modes

this pin should be left disconnected or connected to VA+.

Mode 11 - Digital Data Input & LED Drive

Setup : DP4=1, DP3=0, DP2=1, DP1=0

This is a mode for use with external amplifier and PIN modules that

provide fully digital output levels. An LED is used for transmission

and connected as normal as shown in figure 4. The output from an

external PIN/TIA module is connected to CNT. The polarity of the

input should be such that CNT that goes high when light is

received.

LED current control

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

exceed the LED manufacturer's recommendation for maximum