ACS103 Issue 2.03 May 1996.

ACS103 - 3 Channel, Synchronous or Asynchronous Single Fiber Modem

ACS103

Acapella Optical Modem IC

3 Channel, Synchronous or Asynchronous Single Fiber Modem

Main Features:

Enables three full-duplex serial transmission
channels through a single fiber optic cable,
providing a virtual six fiber path.

Link lengths up to 5 km.

Uses a single Ping Pong LED (PPLED) to both
transmit and receive data.

Supports 7 synchronous data rates up to 64 kbps
or asynchronous data rates from DC to 19.2/
9.6 kbps.

Supports three additional low frequency
asynchronous channels or RS-232 handshake
signals: RTS, CTS, DTR and DSR.

BER 10

-9

Digital mode, allowing the user to add an
external amplifier. Also enables the ACS103 be
used in non-fiber applications e.g. IR/RF.

ACS101 emulation mode giving 100 %
compatibility with the ACS101.

General Description:

The ACS103 is a complete synchronous optical-
modem controller/driver IC, supporting various
user-programmable, full-duplex data-rates to
256 kbps over a single fiber. Alternatively, up to 3
channels of 64 kbps each can be carried. In
'Standard' mode, the fiber may be up to 2.5 km long,
or up to 5 km in 'Double' mode.

Communicating optical-modems automatically
maintain synchronisation with each other such that
the receive phase of one modem is lined-up with the
transmit phase of the other, compensating for the
propagation delay presented by the link. Link lengths
from zero to maximum distance are catered for
automatically.

The ACS103 incorporates ACS101 functionality, with
additional logic to support 3 channels.

Inter-IC Encoding Technique

The 3B4B encoding method is used for
communication between ACS103s, thus ensuring
that there is no DC component in the signal. The
encoding and decoding is transparent to the user.

DCDB RTS CTS DTR DSR XI3 XO3 RSS DR

(1:4)

(1:3)

CKC SEL

XI1 XO1 XI2 XO2

FIFO

Compress

FIFO

Decompress

Digital

Filter

TxD1

TxD2

TxD3

RS-232 Interface

RxD1

RxD3

RxD2

RxCL

TxCL

GND

VA+

VD+

Digital

Filter

LED Driver

LED Receiver

Control Logic

GND

PPLED

LDN

CNT

TRC

LDP

3B4B

Decoder

3B4B

Encoder

XTAL

Block diagram of ACS103

4 3

ACS103 Issue 2.03 May 1996.

ACS101 Emulation mode using "SEL" input

The "SEL" input pin is normally held Low in order to
configure the device in ACS103 mode. If the "SEL"
input pin is allowed to pull High via an internal
resistor the device will be configured in ACS101
emulation mode. In ACS101 emulation mode the
device is fully compatible with the ACS101. The
TxD2, TxD3 and XI3 inputs are internally disabled
with the RxD2, RxD3 and XO3 outputs forced into the
high impedance state. A full-duplex path then exists
between TxD1 and RxD1.

Transmitter and Receiver Functions

The TxD input data of the transmitting modem is
compressed, filtered, and 3B4B encoded. 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 filtered
again to improve BER then directed to the RxD
output pin. Signals TxD and RxD in this specification
refer to the set of signals TxD1/2/3 and RxD1/2/3
respectively.

Transmit Current

The transmit current to the LED can be defined by the
Transmit Current pin (TRC). The current is set to the
maximum value (~100 mA) when TRC is
unconnected. The current is set to the minimum
value (~10 mA) when TRC is connected directly to
ground. To obtain current values between minimum
and maximum, TRC is connected to ground through
a resistor. The resistor value R is given by:

LED current drive (mA) = 100 * (110+R)

(1100 + R)

Data-rate Selection

ACS103 mode SEL = 0

DR4 DR3 DR2 DR1

Data-rate Distance

(kbps)

n/a

9.6

Double

19.2

Double

38.4

Double

n/a

DC - 4.8

Double

n/a

2.4

Standard

9.6

Standard

19.2

Standard

38.4

Standard

48.0

Standard

56.0

Standard

DC - 9.6

Standard

64.0

Standard

n/a = not available

The data rates given in the previous table apply to
TxD1, TxD2 and TxD3.

ACS101 mode SEL = 1

DR4 DR3 DR2 DR1

Data-rate Distance

(kbps)

256.0

Standard

9.6

Double

19.2

Double

38.4

Double

48.0

Double

192.0

Standard

0 DC - 19.2

Double

64.0

Double

2.4

Standard

9.6

Standard

19.2

Standard

38.4

Standard

48.0

Standard

56.0

Standard

0 DC - 38.4

Standard

64.0

Standard

The ACS101 mode is a single channel mode
configuration - the data rates given in the previous
table apply to TxD1 only.

'Standard' mode of operation is up to 2.5 km and
'Double' mode is up to 5 km. The two asynchronous
modes are selected when DR3 = 1, DR2 = 1, DR1= 0.
The above data-rates are obtained when used in
conjunction with a crystal of 9.216 MHz. Non-
standard data-rates may be generated by using the
appropriate crystal frequency. For example, to
generate 160 kbps data-rate in ACS101 mode, use a
crystal frequency of: (160/192) * 9.216 MHz, i.e.
7.68 MHz. Other "non-standard" data-rates may be
generated in the same way as long as the 5 - 10 MHz
crystal range is observed.

For the data-rates 9.6, 19.2, and 38.4 kbps in
ACS103 mode (9.6, 19.2, 38.4, 48 and 64 kbps in
ACS101 mode), the data-rate selection pins DR3-
DR1 are common to 'Standard' and 'Double' modes
of operation with DR4 selecting the mode.

Modem Control Signals

RSS.

The RTS/XI1, CTS/XO1, DTR/XI2, DSR/XO2, XI3
and XO3 signals may be used in either of two modes,
depending on the RSS setting:

RSS = Low; data transmission mode

RSS = High; modem handshake mode

RSS Low - Data Transmission Mode.

When configured in data transmission mode the
inputs XI1, XI2 and XI3 are sampled continuously
with the outputs appearing at XO1, XO2 and XO3
(respectively) of the far-end modem. The sample
frequency for 'Standard' mode is: (crystal freq.)/1536,

ACS103 Issue 2.03 May 1996.

or 6.0 kHz using the recommended crystal frequency
- 9.216 MHz. The sample frequency for 'Double'
mode is: (crystal freq)/3072, or 3 kHz using the
recommended crystal of 9.216 MHz.

The output filters for XO1/2/3 require a minimum
over-sampling of 6 on data presented to the XI1/2/3
inputs.

In ACS101 mode the XI3 input is internally disabled
and the XO3 output is forced into the high impedance
state.

RSS High - Modem Handshake Mode.

In modem handshake mode the control signals are
used as conventional handshake signals between the
DTE (terminal) and the DCE (modem):

DSR (Data Set Ready) DCE

�

DTE.

The DCE is powered up and asserts a Low (active
level) on DSR. The DTE is informed that it is
connected to a "live" DCE.

DTR (Data Terminal Ready) DTE

�

DCE.

The DTE is powered up and asserts a Low (active
level) on DTR. The DCE is informed that it is
connected to a "live" DTE. If DTR is set High, the
DCE responds by taking DCDB High.

RTS (Request to Send) DTE

�

DCE.

The DTE recognises that synchronisation has been
achieved (DCDB active) and asserts a Low (active
level) on RTS. This constitutes a request by the DTE
to send data to the far-end modem.

CTS (Clear to Send) DCE

�

DTE.

The DCE recognises the active RTS signal and
responds by asserting a Low (active level) on CTS. If
RTS is set High the DCE responds by bringing CTS
High.

DCDB (Data Carrier Detect) DCE

�

DTE.

When synchronisation is achieved between DCEs
the DCDB signal is set Low (active level). If
synchronisation is lost the DCE sets DCDB and CTS
High.

Crystal Clock

A crystal may be connected between the pins XLI and
XLO. Alternatively, XLI may be driven directly by an
external clock. The operational frequency range is
5 MHz to 10 MHz, though communicating devices
must be driven at the same nominal frequency with a
tolerance of 100 ppm. In synchronous mode the
frequency should be 9.216 MHz, resulting in the
standard range of synchronous communication
frequencies selected by DR1-DR4.

For asynchronous operation, the choice of crystal
clock frequency dictates the sample rate of the
asynchronous data appearing at the input TxD, and

consequently the jitter on the output RxD. The
sample frequency is always 1/108 of the chosen
clock frequency in 'Standard' mode and 1/216 in
'Double' mode. In ACS101 emulation mode the
sample frequency is 1/36 of crystal clock frequency
in 'Standard' mode and 1/72 of the crystal clock
frequency in 'Double' mode.

Integrating Capacitor

The ACS103 requires the use of an integrating
ceramic capacitor of value 22 nF - 33 nF between
pins CNT and GND for a crystal oscillator frequency
range of 5 - 10 MHz.

DCDB

The Data Carrier Detect (DCDB) signal goes Low
when the modems are locked and ready for data
transmission.

PORB

The PORB pin is a single-pin alternative to the reset
combination DM3 = 0, DM2 = 0, DM1 = 1. If left
unconnected the input pulls High to the operational
state. Selecting reset using DM1-DM3 or holding
PORB Low turns off the LED and most of the digital
logic. The device has been designed to power-up
correctly and operate without the aid of PORB.

Transmission Clock TxCL

The ACS103 gives a choice between internally and
externally generated transmission clocks (see Figure
2. Timing diagrams for set-up and hold
specifications).

When the CKC pin is held Low, TxCL is configured as
an output producing a clock at the frequency defined
by DR1-DR4. Data is clocked into the device on the
rising edge of the internally supplied clock.

When the CKC pin is held High, TxCL is configured
as an input, and will accept an externally produced
transmission clock with a tolerance of up to 500 ppm
with respect to the transmission rate determined by
DR1-DR4. The ACS103 performance is at its best
when external changes on input pins are
synchronised with internal clocks. Therefore,
superior performance is likely when using the
internally generated data transmission clock. If
however, an externally generated transmission clock
is used, then TxCL and TxD are generally
asynchronous to the ACS103 internal clocks,
performance in this case will be enhanced by limiting
the edge speed of the TxCL and TxD signals so that
they are greater than 150 ns. The modem has been
designed to cope with very slow edges on inputs,
without fear of metastability problems.

Receive Clock RxCL

In synchronous mode data is valid on the rising edge
of RxCL clock (see Figure 2. Timing diagrams). To
ensure that the average receive frequency is the

ACS103 Issue 2.03 May 1996.

same as the transmitted frequency RxCL is
generated from a Digital Phase-Lock Loop (DPLL)
system. The DPLL makes periodic corrections to the
output RxCL clock to compensate for differences in
the crystal values, and in the case of an externally
supplied transmission clock (TxCL), compensation is
also made for differences in frequency between this
supplied data clock and the selected clock rate (DR1-
DR4). The DPLL is adaptive and will minimise the
frequency of correction and jitter when the crystal
values and transmission clocks are tightly
toleranced.

If the ACS103 receive FIFO empties (e.g.
transmissions at far-end are halted) the RxCL clock
stops, therefore rising edges of the RxCL clock
always correspond to valid received data bits. This
enables the system designer to use the ACS103 for
the transmission of packets of data with blank periods
between packets. The minimum quanta of data that
can be sent over the link is three bits.

In asynchronous mode the RxCL clock is turned off.

Diagnostic/Operational Modes

The diagnostic/operational modes input pins DM1-
DM3 may be changed asynchronously within a
window of (crystal clock period) * 1536. The
diagnostic mode signals are sampled 1536 * (crystal
clock period) after a change is detected on any of the
DM inputs. The sampled value is taken as the valid
diagnostic mode.

Diagnostic Mode

Lock

DM3 DM2 DM1

Full-duplex

drift

Reset

Remote loop-back

active

Full-duplex

random

Local loop-back

drift

Full-duplex slave

active

Full-duplex

active

Local loopback and remote loop-back are only
available in ACS101 emulation mode.

Full-duplex

In full-duplex configuration the RxCL clock of both
devices tracks the average frequency of the TxCL
clock of the opposing end of the link. The receiving
Digital Phase-Lock Loop (DPLL) system makes
periodic adjustments to the RxCL clock to ensure that
the average frequency is exactly the same as the far-
end TxCL clock. In this mode each TxCL is an
independent master clock and each RxCL a slave
clock.

Full-duplex slave

In slave mode the TxCL and the RxCL clock is
derived from the TxCL clock of the opposing side of
the link, such that the average frequency is exactly
the same. It is therefore essential that only one
modem is configured in slave mode at a time. The

CKC pin is overridden so that TxCL is always
configured as an output.

Local Loopback

Local loopback is only available in ACS101
emulation mode.

In local loopback mode data is looped back inside the
near-end modem and is output at its own RxD output.
The data is also sent to the far-end modem;
synchronisation between the modems is maintained.
In local loopback mode, data received from the far-
end device is ignored, except to maintain lock.
When local loopback is activated the DCDB signal
assumes the logic High state. If concurrent requests
occur for local and remote loopback, local loopback
is selected.

When RSS = 0, RTS and DTR are looped back to
CTS and DSR respectively.

Remote Loopback

Remote loopback is only available in ACS101
emulation mode.

In remote loopback mode the near-end modem
sends a request to the far-end modem to loopback its
received data, thus returning the data. The far-end
modem also outputs the received data at its RxD.
Both modems are exercised completely, as well as
the LEDs and the fiber optic link. Once remote
loopback is established, DCDB on the near-end
(initiating) modem is Low, and DCDB on the far-end
modem is set High. Any data appearing on the TxD
input of the far-end modem is ignored.

When RSS = 0, RTS and DTR are looped back to
CTS and DSR respectively.

Drift lock

Communicating modems attain a stable state where
the "transmit window" of one modem coincides with
the "receive window" of the other allowing for delay
through the optical link. Adjustments to machine
cycles are made automatically during operation to
compensate for differences in crystal frequencies
which cause loss of synchronisation.

Using drift lock, synchronisation described above
depends on a difference in the crystal frequencies at
each end of the link, the greater the difference the
faster the locking. Therefore, if the difference
between crystal frequencies is very small (a few
ppm), automatic locking may take tens of seconds.

Active Lock Mode

Active lock mode may be used to accelerate
synchronisation of a pair of communicating modems.
This mode synchronises the modems with less than
250 ms delay, by adjusting the machine cycle of the
modem. Active lock reduces the machine cycle of
the device by 0.5 % ensuring rapid lock. After

ACS103 Issue 2.03 May 1996.

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

�

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

Drift

Active

Drift

Random

Active

Not allowed

Active

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.

Электронный компонент: ACS103