Perhaps with the exception of standard I/O current and voltage ratings, specifications for I/O circuits are all too often treated as a meaningless listing of numbers. Nevertheless, manufacturers specifications provide valuable information about the correct and safe application of interfaces. These specifications place certain limitations on the module and also on the field equipment that it can operate. Failure to adhere to specifications can result in a misapplication of the hardware, leading to faulty operation or equipment damage. Table 6-6 provides an overview of the electrical, mechanical, and environmental specifications that should be evaluated for each PLC application. Following is a more detailed explanation of each specification. These specifications should also be evaluated for the interfaces covered in the next two chapters (analog and special function).
ELECTRICAL
Input Voltage Rating. This AC or DC value defines the magnitude and type of signal that will be accepted by
the circuit. The circuit will usually accept a deviation from this nominal value of 10 15%. This specification may also be called the input voltage range. For a 120 VAC rated input circuit with a range of 10%, the minimum and maximum acceptable input voltages for continuous operation will be 108 VAC and 132 VAC, respectively.
Input Current Rating. This value defines the minimum input current at the rated voltage that the input device
must be capable of driving to operate the input circuit. This specification may also appear indirectly as the
minimum power requirement.
Input Threshold Voltage. This value specifies the voltage at which the input signal is recognized as being
absolutely ON. This specification is also called the ON threshold voltage. Some manufacturers also specify an OFF voltage, defining the voltage level at which the input circuit is absolutely OFF.
delay is a result of filtering circuitry provided to protect against contact bounce and voltage transients. The input delay is typically 9 25 msec for standard AC/DC inputs and 1 3 msec for TTL or electronic inputs.
Output Voltage Rating. This AC or DC value defines the magnitude and type of voltage source that the I/O
module can control. Deviation from this nominal value is typically 10 15%. For some output interfaces, the output voltage is also the maximum continuous voltage. The output voltage specification may also be stated as the output voltage range, in which case both the minimum and maximum operating voltages are given. An output circuit rated at 48 VDC, for example, can have an absolute working range of 42 to 56 VDC.
Output Current Rating. This specification is also known as the ON-state continuous current rating, a value
that defines the maximum current that a single output circuit can safely carry under load. The output current rating is a function of the electrical and heat dissipation characteristics of the component. This rating is generally specified at an ambient temperature (typically 0 60 C). As the ambient temperature increases, the output current decreases. Exceeding the output current rating or oversizing the manufacturer s fuse
rating can result in a permanent short-circuit failure or other damage.
Output Power Rating. This maximum value defines the total power that an output module can dissipate with
all circuits energized. The output power rating for a single energized output is the product of the output voltage rating and the output current rating expressed in volt-amperes or watts (e.g., 120 V 2 A = 240 VA). This value for a given I/O module may or may not be the same if all outputs on the module are energized simultaneously. The rating for an individual output when all other outputs are energized should be verified with the manufacturer.
Current Requirements. The current requirement specification defines the current demand that a particular I/O
module s logic circuitry places on the system power supply. To determine whether the power supply is adequate, add the current requirements of all the installed modules that the power supply supports, and compare the total with the maximum current the power supply can provide. The current requirement specification will provide a typical rating and a maximum rating (all I/O activated). An insufficient power supply current can result in an undercurrent condition, causing intermittent operation of field input and output interfaces.
Surge Current (Max). The surge current, also called the inrush current, defines the maximum current and
duration (e.g., 20 amps for 0.1 sec) for which an output circuit can exceed its maximum ON-state continuous current rating. Heavy surge currents are usually a result of either transients on the output load or power supply line or the switching of inductive loads. Freewheeling diodes, Zener diodes, or RC networks across the load terminals normally provide output circuits with internal protection. If not, protection should be provided
the control logic goes TRUE is the total of the two program scan times plus the I/O update, output-ON delay, and device-ON response times.
Output-OFF Delay. The output-OFF delay specification defines the response time for the output to go from
ON to OFF once the logic circuitry has received the command to turn OFF. The OFF response time of the output circuit affects the total time required to deactivate an output device. The worst-case time required to turn an output device OFF after the control logic goes FALSE is the total of the two program scan times plus the I/O update, output-OFF delay, and device-OFF response times.
Electrical Isolation. This maximum value in volts defines the isolation between the I/O circuit and the
controller logic circuitry. Although this isolation protects the logic side of the module from excessive input/output voltages or currents, the power circuitry of the module can still be damaged.
Output Voltage/Current Ranges. This specification is a nominal expression of the voltage/current swing of
the D/A converter in analog outputs. This output will always be a proportional current or voltage within the output range. A given analog output module may have several hardware- or softwareselectable, unipolar or bipolar ranges (e.g., 0 to 10 V, 10 to +10 V, 4 to 20 mA).
Input Voltage/Current Ranges. This specification defines the voltage/ current swing of the A/D converter in
analog inputs. This specification will always be a proportional current or voltage within the input range. A given analog input module may have several hardware- or software-selectable, unipolar or bipolar ranges (e.g., 0 to 10 V, 10 to +10 V, 4 to 20 mA).
Digital Resolution. This specification defines how closely the converted analog input/output current or
voltage signal approximates the actual analog value within a specified voltage or current range. Resolution is a function of the number of bits used by the A/D or D/A converter. An 8-bit converter has a resolution of 1 part in 28 or 1 part in 256. If the range is 0 to 10 V, then the resolution is 10 divided by 256, or 40 mV/bit.
Output Fuse Rating. Fuses are often supplied as a part of the output circuit, but only to protect the
semiconductor output device (triac or transistor). The manufacturer carefully selects the fuse that is employed or recommended for the interface based on the fusing current rating of the output switching device.
Fuse rating incorporates a fuse opening time along with a current overload rating, which allows opening within a time frame that will avoid damage to the triac or transistor. The recommended specifications should be followed when replacing fuses or when adding fuses to the interface.
MECHANICAL
Points Per Module. This specification defines the number of input/output circuits that are on a single module
(encasement). Typically, a module will have 1, 2, 4, 8, or 16 points per module. The number of points per module has two implications that may be of importance to the user. First, the less dense (fewer the number of points) a module is, the greater the space requirements are; second, the higher the density, the lower the likelihood that the I/O count requirements can be closely matched with the hardware. For example, if a
module contains 16 points and the user requires 17 points, two modules must be purchased. Thus, the user must purchase 15 extra inputs or outputs.
Wire Size. This specification defines the number of conductors and the largest gauge wire that the I/O
ENVIRONMENTAL