I/O Module Specifications of PLC

Manufacturers’ specifications provide information about how an interface device is correctly and safely used. These specifications place certain limitations not only on the I/O module but also on the field equipment that it can operate. Some PLC systems support hot swappable I/O modules designed to be changed with the power on and the PLC operating. The following is a list of some typical manufacturers’ I/O specifications, along with a short description of what is specified.

Discrete I/O Module Specifications of PLC

1. Nominal Input Voltage: This discrete input module voltage value specifies the magnitude (e.g., 5 V, 24 V, 230 V) and type (AC or DC) of user-supplied voltage that a module is designed to accept. Input modules are typically designed to operate correctly without damage within a range of plus or minus 10 percent of the input voltage rating. With DC input modules, the input voltage may also be expressed as an operating range (e.g., 24–60 volts DC) over which the module will operate.

2. Input Threshold Voltages: This discrete input module specification specifies two values: a minimum ON-state voltage that is the minimum voltage at which logic 1 is recognized as absolutely ON; and a maximum OFF-state voltage which is the voltage at which logic 0 is recognized as absolutely OFF.

3. Nominal Current per Input: This value specifies the minimum input current that the discrete input devices must be capable of driving to operate the input circuit. This input current value, in conjunction with the input voltage, functions as a threshold to protect against detecting noise or leakage currents as valid signals.

4. Ambient Temperature Rating: This value specifies what the maximum temperature of the air surrounding the I/O modules should be for best operating conditions.

5. Input On/Off Delay: Also known as response time, this value specifies the maximum time duration required by an input module’s circuitry to recognize that a field device has switched ON (input ON-delay) or switched OFF (input OFF- delay). This delay is a result of filtering circuitry provided to protect against contact bounce and voltage transients. This input delay is typically in the 9 to 25 millisecond range.

6. Output Voltage: This AC or DC value specifies the magnitude (e.g., 5 V, 115 V, 230 V) and type (AC or DC) of user-supplied voltage at which a discrete output module is designed to operate. The output field device that the module interfaces to the PLC must be matched to this specification. Output modules are typically designed to operate within a range of plus or minus 10 percent of the nominal output voltage rating.

7. Output Current: These values specify the maximum current that a single output and the module as a whole can safely carry under load (at rated voltage). This rating is a function of the module’s components and heat dissipation characteristics. A device drawing more than the rated output current results in overloading, causing the output fuse to blow.

As an example, the specification may give each output a current limit of 1 A. The overall rating of the module current will normally be less than the total of the individuals. The overall rating might be 6 A because each of the eight devices would not normally draw their 1 A at the same time. Other names for the output current rating are maximum continuous current and maximum load current.

8. Inrush Current: An inrush current is a momentary surge of current that an AC or DC output circuit encounters when energizing inductive, capacitive, or filament loads. This value specifies the maximum inrush current and duration (e.g., 20 A for 0.1 s) for which an output circuit can exceed its maximum continuous current rating.

9. Short Circuit Protection: Short circuit protection is provided for AC and DC output modules by either fuses or some other current-limiting circuitry. This specification will designate whether the particular module’s design has individual protection for each circuit or if fuse protection is provided for groups (e.g., 4 or 8) of outputs.

10. Leakage Current: This value specifies the amount of current still conducting through an output circuit even after the output has been turned off. Leakage current is a characteristic exhibited by solid-state switching devices such as transistors and triacs and is normally less than 5 milliamperes. Leakage current is normally not large enough to falsely trigger an output device but must be taken into consideration when switching very low current sensitive devices.

11. Electrical Isolation: Recall that I/O module circuitry is electrically isolated to protect the low-level internal circuitry of the PLC from high voltages that can be encountered from field device connections. The specification for electrical isolation, typically 1500 or 2500 volts AC, rates the module’s capacity for sustaining an excessive voltage at its input or output terminals. Although this isolation protects the logic side of the module from excessive input or output voltages or current, the power circuitry of the module may be damaged.

12. Points per Module: This specification defines the number of field inputs or outputs that can be connected to a single module. Most commonly, a discrete module will have 8, 16, or 32 circuits; however, low-end controllers may have only 2 or 4 circuits. Modules with 32 or 64 input or output bits are referred to as high-density modules.

Some modules provide more than one common terminal, which allows the user to use different voltage ranges on the same card as well as to distribute the current more effectively.

13. Backplane Current Draw: This value indicates the amount of current the module requires from the backplane. The sum of the backplane current drawn for all modules in a chassis is used to select the appropriate chassis power supply rating.

Analog I/O Module Specifications of PLC

1. Channels per Module: Whereas individual circuits on discrete I/O modules are referred to as points, circuits on analog I/O modules are often referred to as channels. These modules normally have 4, 8, or 16 channels. Analog modules may allow for either single-ended or differential connections. Single-ended connections use a single ground terminal for all channels or for groups of channels. Differential connections use a separate positive and negative terminal for each channel. If the module normally allows 16 single-ended connections, it will generally allow only 8 differential connections. Single-ended connections are more susceptible to electrical noise.

2. Input Current/Voltage Range(S): These are the voltage or current signal ranges that an analog input module is designed to accept. The input ranges must be matched accordingly to the varying current or voltage signals generated by the analog sensors.

3. Output Current/Voltage Range(S): This specification defines the current or voltage signal ranges that a particular analog output module is designed to output under program control. The output ranges must be matched according to the varying voltage or current signals that will be required to drive the analog output devices.

4. Input Protection: Analog input circuits are usually protected against accidentally connecting a voltage that exceeds the specified input voltage range.

5. Resolution: The resolution of an analog I/O module specifies how accurately an analog value can be represented digitally. This specification determines the smallest measurable unit of current or voltage. The higher the resolution (typically specified in bits), the more accurately an analog value can be represented.

6. Input Impedance and Capacitance: For analog I/Os, these values must be matched to the external device connected to the module. Typical ratings are in Meg-ohm (M Ω) and pico-farads (pF).

7. Common-Mode Rejection: Noise is generally caused by electromagnetic interference, radio frequency interference, and ground loops. Common-mode noise rejection applies only to differential inputs and refers to an analog module’s ability to prevent noise from interfering with data integrity on a single channel and from channel to channel on the module. Noise that is picked up equally in parallel wires is rejected because the difference is zero. Twisted pair wires are used to ensure that this type of noise is equal on both wires. Common-mode rejection is normally expressed in decibels or as a ratio.

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