Euresys Coaxlink 1629, Hardware Manual

Page: 72 / 75

72
Voltage levels and margins in a TTL (5 V) system, R = 180 ohm
Isolated
Output
Logic Level
Isolated Output
State
Isolated Output
Voltage Level
TTL Input
Voltage
Level
Voltage
Margin
TTL Input
Logic
Level
High Close
4.1 V max
(2)
2.0 V min 2.1 V High
Low Open
0.36 V max
(1)
0.8 V max 0.44 V Low
Voltage levels and margins in a LVTTL (3.3 V) system, R = 180 ohm
Isolated
Output
Logic Level
Isolated Output
State
Isolated Output
Voltage Level
TTL Input
Voltage
Level
Voltage
Margin
TTL Input
Logic
Level
High Close
2.4 V min
(2)
2.0 V min 0.4 V High
Low Open
0.36 V max
(1)
0.8 V max 0.44 V Low
See also: "Isolated Output" on page 49 for voltage levels of isolated outputs.
NOTE
(1) 0.36 V is obtained considering a worst-case external (pull-up) load of 2
mA ( 180 ohm x 2 mA = 0.36 V), which means that the circuit can support the
presence of an external pull-up resistor up to a (minimum) value of 1K5 ohm
(in 3.3 V) or 2K4 ohm (in 5 V). If needed, an other R value can be chosen
according to the actual pull-up load within the circuit.
NOTE
(2) In any case, the voltage drop across the opto-coupler pins (V
OUT
+
- V
OUT
t
-
) is lower than 0.9 V. Which gives the following results: 3.3 V – 0.9 V = 2.4 V; 5
V – 0.9 V = 4.1 V.
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.
Coaxlink Hardware Manual