Detailed Description

Principles of Operation

Input Current Clamp

The input pins (IN1–IN8) sense the state (on versus off)

of field sensors by monitoring both the voltage and the

current flowing through the sensor output. The current

sinking through these input pins rises linearly with input

voltage until the limit set by the current clamp is reached.

Any voltage increase beyond this point does not increase

the input current any further.

The value of the current clamp is adjustable through

an external resistor connected between pin RIREF and

ground. The voltage at the input pins (IN1–IN8) are com-

pared against internally set references to determine if the

sensor is on (logic 1) or off (logic 0). The trip points deter-

mining the on/off status of the sensor can be selected

through pin VTSELECT as follows:

● VTSELECT = 1 selects trip points that satisfy the

requirements of IEC 61131-2 type 1, type 2, and type 3

switches.

● VTSELECT = logic 0 selects trip points that are CMOS

logic compatible and roughly centered approximately

2.5V.

Pins RT1–RT8 must be connected directly to GND to

provide a return path for the input current if LEDs are

not required for visual indication. If VTSELECT = logic

0, RT1–RT8 must be connected directly to GND. If

visual indication is needed when VTSELECT = logic 0,

then LEDs can be connected to pins OP1–OP8 through

external current-limiting resistors.

PIN

NAME

FUNCTION

1, 3, 6, 18, 21

N.C.

No Connection

2

VTSELECT

Selects input trip points to be CMOS or IEC 61131-2 compliant.

Logic 0 = CMOS compliant

Logic 1 = IEC 61131-2 compliant

4

DB0

Debounce (Filtering) Time Select Inputs

5

DB1

DB1

DB0

TIME CONSTANT OF FILTER APPLIED

0

0

0 (no filtering)

0

1

0.025ms

1

0

0.75ms

1

1

3ms

7, 9, 11, 13, 15,

25, 27, 29

IN1–IN8

Field Input n. IN1 is pin 7 and IN8 is pin 29.

8, 10, 12, 14,

16, 24, 26, 28

RT1–RT8

Energyless LED Driver Outputs. Connect to GND if LEDs are not required. RT1 is pin 8 and

RT8 is pin 28.

17

RIREF

Current-Limiter Reference Resistor

19

VCC24V

Field-Supply Voltage

20

5VOUT

5V Regulator Output

22

UVFAULT

Indicates Low Supply Voltage Alarm (Active Low)

23

FAULT

Indicates Hot Temperature Alarm. This is OR’ed with the UVFAULT indicator (active low).

30–37

OP8–OP1

Logic Output n. OP1 is is 37. OP8 is pin 30.

38

GND

Field Ground

—

EP

Exposed Pad. Must connect EP to the PCB ground plane.

MAX31915

Industrial, Octal, Digital Input Translator

www.maximintegrated.com

Maxim Integrated │ 9

Pin Description