UG125 Rev.1.00
Page 3 of 10
Aug 24, 2017
ISL32740EVAL2Z
2. Functional Description
2.
Functional Description
The ISL32740E is an isolated, 40Mbps, half-duplex PROFIBUS transceiver. The device incorporates a patented refresh
circuit to maintain the correct output state with respect to data input. At power-up, the bus outputs follow the
Truth Tables
.
The DE input should be held low during power-up to prevent false drive data pulses on the bus. This is accomplished by
connecting DE through a 10kΩ pull-down resistor to GND1.
This transceiver uses a differential input receiver for maximum noise immunity and common-mode rejection. Input
sensitivity is ±200mV, as required by the RS-485 specification.
The receiver input resistance meets the PROFIBUS “Unit Load” (UL) requirement of 54kΩ minimum. The receiver
includes a “fail-safe if open” function that ensures a high-level receiver output if the receiver inputs are unconnected
(floating). The receiver output is tri-statable through the active low RE input.
The PROFIBUS driver is a differential output device that delivers at least 2.1V across a 54Ω purely differential load.
The driver features low propagation delay skew to maximize bit width and to minimize EMI.
The driver in the ISL32740E is tri-statable using the active high DE input. The outputs of the ISL32740E driver are not
slew rate limited, so faster output transition times allow data rates of at least 40Mbps.
Important Notice:
For convenience, an isolated DC/DC converter is used to provide the power supply across the isolation barrier. Its
isolation ratings, however, are lower than those of the transceiver. Therefore, ensure that the ground potential
difference between the left side (GND1, GND2) and the right side (GND3, GND4) of the board remain below 50VAC
(see
Figure 2 on page 4
).
2.1
Truth Tables
Note: *Transceiver shutdown mode.
Receiving
Inputs
Outputs
RE
DE
A-B
RO
0
0
V
AB
≥ -0.05V
1
0
0
-0.05 > V
AB
> -0.2V
Undetermined
0
0
V
AB
≤ -0.2V
0
0
0
Inputs Open/Shorted
1
1
1
X
High-Z
1
0
X
High-Z*
Transmitting
Inputs
Outputs
RE
DE
D
ISODE
B
A
X
1
1
1
0
1
X
1
0
1
1
0
0
0
X
0
High-Z
High-Z
1
0
X
0
High-Z*
High-Z*
