Module Operation

A network of resistors, capacitors, and zener diodes establishes input thresholds and provides

input filtering. Optoisolators provide isolation between the field inputs and the module’s logic

components. An oscillator and switch form a sampling circuit that is transparent to the

controller and the LEDs that indicate the state of the inputs. Data from all 16 inputs is placed

into a data buffer. The module’s circuit LEDs show the current states of the 16 inputs in this

data buffer.

Parallel-to-serial converters change input data from the data buffer into the serial format

needed by the Bus Interface Unit.

After checking the Board ID and verifying that the module is receiving appropriate logic power

from the Bus Interface Unit (which is reflected by the state of the module’s Power LED), the Bus

Interface Module then reads the filtered, converted input data.

Positive or Negative Inputs

Inputs for this module can be either positive or negative (all 16 inputs must be the same type).

Both types of signal produce a logic 1 (true) when the switch is closed. Selection of positive or

negative operation is made by the manner in which the external power supply is connected to

the inputs and to the I/O Terminal Block.

Positive inputs receive current from input devices and supply current to the common or

negative power bus. Input devices are connected between the positive power bus and the input

terminals.

Negative inputs provide current to input devices and accept current from the common or

positive power bus. Input devices are connected between the negative power bus and the input

terminal.

Field Wiring

The following illustration shows terminal assignments for the 48 VDC Positive/Negative

Input Module.