PLC I/O module selection steps



General prices accounted for more than half the price of PLC I/O modules. PLC I/O modules switch I/O module, analog I/O modules and special function modules. Different I/O modules, the circuit and function is different and directly affect the price of the application of PLC and should be selected according to the actual need.


(A) the switch I/O module option


1. the choice of switching input module


Switching input module is used to receiving input device for switching signal, the signal is converted to a PLC accepts lowvoltage signals, and achieve electrical isolation for PLC, the external signal. Options should mainly consider the following aspects:


1) input signal type and voltage levels


Switching input module with DC input and AC input and AC/DC input three types. Choice mainly according to the input signal and environmental factors. Shorter delay time of DC input module, you can also directly with the proximity switches, photoelectric switches and other electronic input devices; AC input module reliability, suitable for the harsh environment of oil mist and dust.


Switching input module input signal voltage: DC 5V, 12V, 24V, 48V, 60V; AC 110V and 220V. Selection is mainly based on field input device and enter the distance between the modules to take into account. 5V, 12V, 24V for transport distance, such as 5V input module to not more than 10 meters. Far higher input voltages of the module should be used.


2) input connection


Switching input module main meeting point and group two connection modes, as shown in Figure 6-2.


Figure 6-2 switching input module wiring


A) meeting point input b) grouping input


Meeting point of switching input module all input points share a common Terminal (COM), whereas the group switch quantity input module is the entry point into a number of groups, each group (several entry points) have a common Terminal, is a separation between the groups. Group switch quantity input prices higher than the meeting point of the module, if the input signal does not require a separation between general type of selection meeting point.


3) Note number and the connected input


For selection of high density input modules (32 points, 48 points), you should consider the module to connect the points generally do not exceed 60% of the input numbers.


4) input threshold level


In order to improve the reliability of the system, you must consider the size of the input threshold level. Threshold level, the higher the stronger anti-interference ability, the transmission distance farther, specific, see the PLC instructions.


2. digital output module selection


Digital output modules are PLC internal low voltage signal into a drive outside of switch signal output device, and achieve electrical isolation for PLC signals. Options should mainly consider the following aspects:


1) output mode


Digital output module with relay output, transistor and thyristor output output in three ways.


Following electrical output of price cheap, both can for drive Exchange load, and can for DC load, and applies of voltage size range more wide, and guide pass pressure drop small, while bear instantaneous had voltage and had current of capacity strong, but its belongs to has touch points components, action speed more slow (drive perceptual load Shi, touch points action frequency shall not over 1HZ), and life more short, and reliability poor, only applies Yu not frequently pass broken of occasions.


For frequent switching of the load, you should use thyristor or transistor output, contactless symbol they belong to. Thyristor output can only be used for AC load, and can only be used for the DC transistor output.


2) output connection


Digital output modules are grouped and separated two connection modes, as shown in Figure 6-3.



Figure 6-3 digital output module wiring

A) grouping output b) delimited output


Grouping output is output as a set, a group have a common Terminal, is a separation between the groups, can be used to drive external output devices of different power; separate outputs each output point is there is a common Terminal, the mutual isolation between output points. Select primarily on the basis of PLC output units of power type and voltage depending on how many. General integral PLC group output, separate output.


3) drive capability


Switch output current output module (drive) must be greater than PLC rated current of an external output device. Users should be based on actual current size to select the output device output current output module. If the actual output current is large, output modules cannot be directly driven, can increase the enlargement process.


4) Note number and connected to the output


Select digital output module, the number of simultaneously connected to the output points should also be considered. While connected output device of aggregate of allowable current value must be less than the public side of the current value, such as a 220V/2A 8 point output module, each output can withstand current of 2 a, but the currents are not allowable output common 16A (8×2A), is usually much smaller than this value. In General, and access points do not exceed 60% of the same common Terminal output points.


5) the maximum output current and load type, ambient temperature and other factors


Switch output module specifications, it is closely related to different types of loads, in particular the maximum current output. In addition, the maximum output current of thyristor increases with rising temperature will be lower, in actual use should also pay attention.


(B) selection of analog I/O modules


Main function is data conversion of analog I/O modules, and connected to the PLC internal bus, and for the safe electrical isolation. Analog inputs (a/d) module is produced will be detected by the sensor, the continuous analogue signal into a PLC within acceptable digital; analog output (d/a) module is a PLC within the digital convert analog signal output.


Typical measuring range of the analog I/O modules -10V~+10V, 0~+10V, 4~20mA, etc, can be selected according to the actual need, but should also take into account such factors as its resolution and accuracy.


PLC manufacturer also offers a special analog input module, can be used to directly receive low level signals (such as RTD, thermocouple, etc).


(C) the selection of special function modules


At present, the PLC manufacturer have launched a number of I/O modules with special functionality, and some also launched its intelligent I/O modules with CPU, high speed counter, cam emulators, position control, PID control module, communication modules, and so on.

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