Alkeria NECTA Series - I;O Switching Time; Bidirectional Module Structure (Port 0); To 24 V Output

4. Interfacing to the world 55
4.3.2.4 5 to 24 V Output
User Device
User GND
Input+
10K
24 Vdc
Figure 4.16: Connecting output port P3+ to a 24 V input
NECTA with 24 V tolerant I/O interface can drive also 5 to 24 V inputs using the connections shown in
Figure 4.16: the ‘0’ logic level is generate by the output driver ”shorting” the external pullup resistor
to ground, while the ‘1’ level is imposed by the external pullup voltage. This conguration needs to be
enabled using the procedure in Section 4.5.2.
External pullup resistor value must be calculate in order to maintain the 0 V output current under 30 mA,
as shown below:
R
P ullup
[kΩ] ≥
V
P ullup
[V]
30 mA
(4.1)
Note that the lowest pullup value, the slowest commutation time, the highest ’0’ logic voltage value.
4.3.3 Bidirectional module structure (port 0)
NECTA port 0 can be software-congured either as an input or as an output. When the camera is pow-
ered on, the port is initially congured as an input (termination disabled); it can be set as an output by
software. All the statements above about input and output ports, including methods to interface to
signals with logic levels different from RS-422, apply also to port 0. For more information on how to
congure and use the input/output port refer to the code samples in Section 4.5 and to the examples
found in the MaestroUSB3 SDK.
4.3.4 I/O switching time
As reported in the I/O schematics shown in Figure 4.4-4.5 and below, NECTA cameras use RS-422 differ-
ential transceivers mod. LTC2855-LTC2865, made by Linear Technology. They allow very short switch-
ing time: the input propagation delay, due to the input receiver, is less than 70 ns, the output delay is
less than 50 ns
1
; both rise and fall time are less than 12.5 ns.
1
Valid only for push-pull output congurations. When the output is open-collector, delay depends on the external pull-up.