for each clock cycle. On one edge, the transmit data output changes. On the other edge, the receive
data input is sampled. The assignment of clock edges to transmit data transition and receive data
sampling is referred to as clock polarity.
The SyncLink USB uses the clock polarity in the RS-232/RS-422/V.24/V.28/V.11 standards as described
below:
RS-232/V.28 Single Ended Signals
+3V to +15V (+5V typical) = clock on
-3V to -15V (-5V typical) = clock off
On to Off edge (falling edge) = receive data sample (bit center)
Off to On edge (rising edge) = transmit data transition (bit edge)
RS-422/RS-485/V.11 Differential Signals
+200mV to +6V (+2V typical) = clock on
-200mV to -6V (-2V typical) = clock off
On to Off edge (falling edge) = receive data sample (bit center)
Off to On edge (rising edge) = transmit data transition (bit edge)
Most serial communications equipment uses the above clock polarity, but some non-standard
equipment may use the opposite polarity. For differential signals, inverting the conductors of each clock
signal pair will alter the polarity.
Serial Interface Selection
The serial adapter supports different interface types, which are selected by software configuration. By
default on power up, the interface is disabled and does not drive any outputs. The method of changing
the interface type depends on the operating system and application. For example, Windows implements
this setting in the device properties of the device manager and Linux implements the setting with an
ioctl() call. Refer to the software documentation for details.
Some interface types require a conversion cable in addition to a software setting to provide the
necessary connector type. Refer to the Serial Pin Assignments section for more details.
Differential Input Termination
The port on the card has 120-ohm termination of RS-422 differential inputs. Termination is used to
increase signal reliability at high speeds (generally 1Mbps or more). At high speeds, receivers at each
end of a cable should be terminated.
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