Using Local 12 V Power
The power supply voltage is taken from the I/O connector itself, using the power supply pin “+12V”.
This circuit needs two resistors, named R and RPOL.
A resistor of 180 ohm 1/8W is suggested for R, as best compromise but the circuit can also work within a large range of resistor values from 50 ohm 1/2W to 10K ohm 1/16 W (1).
A resistor of 560 ohm 1/4W is suggested for RPOL, as best companion of R = 180 ohm but the value of RPOLcan be adapted to match accordingly others R values.
If an existing pull-down resistor is already available at the TTL receiver side it can be used as R resistor to operate the circuit, avoiding the need of adding an extra resistor somewhere in the cabling.
The circuit does not perform logic level inversion.
Note: The resistor value can be also changed to match special static or dynamic performance requirements.
Note: The circuit does not perform logic level inversion.
Wiring Diagram
Refer to Connectors for I/O connectors pin assignments.
| 1. | Connect OUT+ to +12V through a resistor (RPOL) of 560 ohm 1/4 W (or another resistor value that suits the circuit requirements). |
| 2. | Connect OUT- to the TTL input. |
| 3. | Pull-down OUT- with a resistor (R) of 180 ohm 1/8W (or another resistor value that suits the circuit requirements). |
Static Levels Compatibility
The following table shows that the voltage levels are well compatible and that they remains acceptable voltage margins for both TTL and LVTTL applications.
Voltage levels and margins, Rpol = 560 ohm 1/4 W, R = 180 ohm 1/8W.
|
Isolated Output |
Isolated Output |
Isolated Output |
TTL Input |
Voltage Margin |
TTL Input |
|---|---|---|---|---|---|
|
High |
Close |
2.7 V max (2) |
2.0 V min |
0.7 V |
High |
|
Low |
Open |
0.36 V max (1) |
0.8 V max |
0.44 V |
Low |
Refer to Isolated Output for voltage levels of isolated outputs.
Note 1 0.36V is obtained considering a worst-case external (pull-up) load of 2 mA ( 180 ohm x 2 mA = 0.36V), which means that the circuit can support the presence of an external pull-up resistor up to a (minimum) value of 1K5 ohm (in 3.3V) or 2K4 ohm (in 5V). If needed, an other R value can be chosen according to the actual pull-up load within the circuit.
Note 2 RPOLlimits the Voh voltage to about 2.7V in order to match TTL and LVTTL levels.
2.7 V is obtained considering the RPOL-R 560 ohm-180 ohm divider and taking into account that the voltage drop across the opto-coupler pins (VOUT+ - VOUTt- ) is about 0.9V.
Dynamic Limitations
The maximum pulse width of isolated outputs is about 5 µs and the maximum pulse rate is 100 KHz,
Isolated outputs add an extra delay of about 5 µs in the signal propagation.
The resistor value of R = 180 ohm has good dynamic results for a usual capacitive loads as 1 or 2 meters of cable. As example, a 2m cable will add 100 pF of load (50pF/m) which give a rise time of about 18 µs at 180 ohm ( R x C = 180 ohm x 100 pF = 18 µs ) . If needed, the R value can be adapted to match special requirements in terms of rise time and/or capacitive load.
If maximizing the opto-coupler switching time is a concern, it is not recommended to not increase too much the value of the resistor. The opto-coupler circuit behaves better (switching times) with a load of about 10 mA or higher. R = 180 ohm loads the opto-coupler at 13 mA (3.3V) and 23 mA (5V).