204
Connecting TTL Devices to Isolated Output Ports
Interfacing a device with a TTL or LVTTL receiver using an isolated output port
Power must be provided to the opto-coupler transistor in order to operate the circuit.
Two cases are considered:
See also:
"Using External Power" on page 204
when an external 5 V or 3.3 V power supply line
is available and can be carried to the opto-coupler(s) V
out
+ pin(s).
See also:
"Using Local 12 V Power" on page 207
when the power is taken from the board
itself, namely through the +12 V power line connector pin.
Using External Power
The power supply voltage is not taken from the board but comes from the “external” system. A
3.3 V or also 5 V power supply can be considered, as most LVTTL input receiver circuits support
5 V levels at their inputs. The power supply line must be carried through the cable up to the
OUT+ pin of the opto-coupler.
In this case the voltage rail is called V
CC
, as the voltage could be the same as the TTL receiver
V
CC
pin.
This circuit needs only one pull-down resistor as show in the next figure. A resistor of 180 ohm
1/8 W is suggested as best compromise but the circuit can also work within a large range of
resistor values from 50 ohm 1/2 W to 10K ohm 1/16 W (1).
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.
TIP
The resistor value can be also changed to match special
or
"Dynamic limitations " on page
performance requirements. This topic is covered in the coming
paragraphs.
eGrabber
Hardware Manual
