VISTEK V1679 single-mode fibre
data transceiver
HU-V1679
9
When driving the Opto-Isolator from an external voltage source, one must check that the forward current
through the Light Emitting Diode (LED) inside the Opto-Isolator does not exceed 60mA. Otherwise, the Opto-
Isolator can be permanently damaged or even destroyed.
The current
I
F
through the LED is determined by the Input Voltage
V
IN
supplied between a GPIOn+ and
GPIOn- connector pin pair, the forward voltage of the LED
V
F LED
, the forward voltage of the protection diode
V
F 1N4148
(which is in series with the LED), and the on-board 100Ohm current limiting resistor. The reason for
having an extra protection diode is because of the relatively low reverse (breakdown) voltage of the LED,
which is 6V (typ), compared to 75V (min) of a 1N4148 silicon diode. This makes the GP Inputs fairly robust
against accidentally reverse biased input pins.
By applying Ohm's law, the forward current I
F
through the LED can be calculated as follows:
Ω
+
−
=
100
)
(
1N4148
F
LED
F
IN
F
V
V
V
I
Considering the worst-case scenario, by taking V
F LED
(min) and V
F 1N4148
(min), V
IN
must not exceed 7.5V,
unless an extra current limiting resistor in series to the 100 Ohm is provided.
WARNING!
The maximum, continuous current through the Opto-Isolator's LED must not exceed 60mA!
Using the on-board generated forward bias supply for the LED (+5V or +12V), the current through the LED is
limited to about 5mA. This means that driving an Input with an Open Collector Driver for instance, the Driver
must be capable of sinking at least 5mA!
The silk screen on the PCB identifies each Input by its Group Name (A or B) and its number (1..8).
The configuration Jumpers for each GP Input are located closely to the corresponding Opto-Isolator IC. The
Jumpers are positioned as follows:
Short-to-GND
Jumper
Opto-Isolator
5V
1
2
V
A
5
On-board
Input Bias Voltage
Jumper
GPI Input
Group Name & Number
Board Layout
Figure 3 : General Purpose Input Jumper Locations