Advanced Communications
ACS104A Data Sheet
programmed using pins HD(1:2) on the TQFP44 package, in
accordance with the Table 2.
Local Loopback
In local loopback mode TxD data is looped back inside the near-end
modem and appears at its own RxD output. RTS, DTR and RII are
also looped back appearing at their own CTS, DSR and RIO outputs
respectively. The data is also sent to the far-end modem and
synchronisation between the modems is maintained.
HD2
HD1
Handshake
Bandwidth
Skew
w.r.t. RxD
0
0
1
1
0*
1
0
600
10
5
Hz
10 ms.
kHz
kHz
kHz
1 - 2 data bits
1 - 2 data bits
1 - 2 data bits
In local loopback mode data received from the far-end device is
ignored, except to maintain lock. If concurrent requests occur for
local and remote loopback, local loopback is selected.
1
2.5
Table 2. Handshake signal bandwidth allocation
* When HD2 = HD1 = 0 super-compress mode is selected. See
section headed Super-Compress mode.
The local loopback diagnostic mode is used to test data flow up to,
and back from, the local ACS104A and does not test the integrity of
the link itself, i.e. local loopback operates independently of
synchronisation with a second modem.
Handshake data rates which exceed the allocated bandwidth will
be delayed, and consequently result in additional skew between
handshake signals and data.
Remote Loopback
The HD pins enable the user to allocate a maximum bandwidth to
the handshake signals and thus limit the power consumption of the
device. The power consumption is, however, dependent on the
actual bandwidth used and not the bandwidth selected. For
example; if the handshake signals were toggled at 1kHz the power
consumption would be the same for an allocated bandwidth of
2.5kHz as it would for an allocation of 10kHz. See section headed
Current and Power Consumption for more details.
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 so that it appears at the RxD of the initiating modem. RTS,
DTR and RII follows the same path, returning data back to CTS,
DSR and RIO respectively of the initiating modem. Data also
appears at the far-end modem outputs RxD, CTS, DSR and RIO. In
the process both modems are exercised completely, as well as the
LED/PINs and the fiber optic link. The remote loopback test is
normally used to check the integrity of the entire link from the near-
end (initiating) modem. Whilst a device is responding to a request
for remote loopback from the initiating modem (far-end), requests to
initiate remote loopback will be ignored.
Super-Compress mode
2
This mode is selected when HD2 = HD1 = 0. Super-compress
mode performs a second stage of data compression, thus further
reducing the power consumption of the modem. Normally, data is
compressed in a manner which is independent of the data type. In
super-compress mode, an additional stage of compression further
reduces the data by a factor of 1 to 3 depending on the data itself.
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 XTAL frequencies which would
otherwise cause loss of synchronisation.
Example: The super-compress stage will compress DC data by an
additional Compression Factor (CF) of 3, whilst data close to the
maximum frequency will not be compressed beyond the standard
CF of 1.
Using drift lock, synchronisation described above depends on a
difference in the XTAL frequencies at each end of the link, and the
greater the difference the faster the locking. Therefore, if the
difference between XTAL frequencies is very small (a few ppm),
automatic locking may take tens of seconds or even minutes.
Super-compress mode provides benefits where the user is
interested in low average power consumption (e.g. battery life)
rather than peak power. If the intended system is idle for most of
the time with periodic bursts of activity, the additional data
compression afforded will approach a CF of 3.
Drift lock will not operate if the two communicating devices are driven by
a clock derived from a single source (i.e. tolerance of 0ppm).
Locking
Active Lock Mode
To achieve low power consumption the ACS104A is active for a
small percentage of the frame (machine-cycle) known as the
'transmit' window and the 'receive' window, collectively these
windows are known as the 'active time'. Outside the 'active time'
the device is largely dormant accept for the maintenance of the
oscillator and basic 'house-keeping' functions.
Active lock mode may be used to accelerate synchronisation of a
pair of communicating modems. This mode synchronises the
modems in less than 3 seconds by adjusting the machine cycles of
the modems. Active lock reduces the machine cycle of the device
by 0.5 % ensuring rapid lock. After synchronisation the machine
cycle reverts automatically to normal.
Communicating modems attain a stable state known as 'locked',
where the 'transmit' window of one modem coincides with the
'receive' window of the other, allowing for the delay through the
Only one device may be configured in active lock mode at any one
time. Active lock mode is usually invoked temporarily on power-up.
This can be achieved on the ACS104A by connecting DM1 to an RC
arrangement, i.e. with the capacitor to 5V and the resistor to GND, to
create a 5V à 0V ramp on power-up. The RC time constant should
be Ca. 5 seconds. Active lock will succeed even when
communicating devices are driven from clocks derived from a single
source (0ppm).
optical link.
Adjustments to machine cycles are made
automatically during operation, to compensate for differences in
XTAL frequencies which cause loss of synchronisation.
The ACS104A locking algorithm is statistical, and consequently the
locking time will differ on each attempt to lock.
Random Lock
Diagnostic and Locking Modes
This mode achieves moderate locking times (typically 5 seconds,
worst case 10 seconds) with the advantage that the ACS104A’s are
configured as peers. Communicating modems may be permanently
configured in this mode by hard wiring the DM pins.
The diagnostic and operational modes, shown in Table 3, are
selected using the DM pins. DM3 is held high internally on the
PLCC28 package.
DM3
DM2
DM1
Mode
Lock
Random lock will succeed even when communicating devices are
driven from clocks derived from a single source (0ppm). Random
lock mode is compatible with drift lock and active lock.
0
0
0
1
1
1
0
0
1
0
1
1
0
1
0
1
0
1
Full-duplex
Full-duplex
Full-duplex
Local loopback
Remote loopback
Full-duplex
Drift
Active
Memory
Random
Random
Random
Memory Lock
Following the assertion of a reset (PORB = 0) communicating
devices will initiate an arbitration process where within 10 seconds
Table 3. Diagnostic and operational modes
3
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