Digital I/O design for industrial PLC

IEC 61131-2 specifies three voltage/current operating ranges (types 1, 2, and 3) for the digital input modules used in the PLC. In the case of TLP2363, type 1 of voltage/current operating range can be applied by following the circuit constants described below. As shown in Fig. 1, the feature of requirement for digital input is that it defines the area that the module should be turned off. 

The configuration of the digital input module is shown in Figure 2. R₁ and R₂ must be configured to follow the operating area shown in Fig. 1. In conventional high-speed communication photocouplers, only the maximum value is guaranteed for the threshold input current that controls the high/low output, making it difficult to design a module that conforms to the operating area shown in Fig. 1.

TLP2363 ensures not only the maximum threshold input current but also the minimum value, which contributes to simplifying the design of the input module according to the operating area shown in Fig. 1.

In Fig. 2, by using peripheral components with following constants, the operating area shown in Fig. 1 can be covered under the ambient temperature -40°C to 105°C that TLP2363 ensure the operation. ^[Note2]

• R₁ = 2200 Ω
• R₂ = 750 Ω
• R_L = 1000 Ω
• V_CC = 3.3 V

In addition to the above requirements for the operating area of the input circuit, IEC 61131-2 provides provisions for studies to be performed on the behavior of the PLC system with respect to variations in the external supply power. The gradual shutdown and start-up test of the external power supply is required to confirm that the specified behaviors are shown in advance when the power supply is turned on or off for 60 s.

TLP2363 does not need the Schmitt trigger shown in Fig. 2 (Fig. 4) because the output is stable without chattering even when the power supply is turned on or off for 60 s (Fig. 3). This contributes to reducing the number of component parts.

Fig. 3   Left: V_O behavior when starting V_CC from 0V to 3.3V for 60 s
Right: V_O behavior when V_CC falls from 3.3V to 0V for 60 s
I_F: Current flowing on the primary side (LEDs) of the photo coupler
V_O: Output voltage

In addition, TLP2363 has a high tolerance against gradual rise and fall of the input signal so that stable output can be maintained even when the external input is unstable. (Fig. 5)

Fig. 5 Left: V_O behavior when starting I_F from 0mA to 2.4mA for 60 s
Right: V_O behavior when falling I_F from 2.4mA to 0mA for 60 s
I_F: Current flowing on the primary side (LEDs) of the photo coupler
V_O: Output voltage

Since TLP2363 is an open-collector output type, the output logic is as follows when a voltage higher than the minimum operating voltage is applied to V_CC.

From the above, TLP2363 having 15 Mbps (typical) data transmission is suitable for the design of digital input modules of PLC that require high-speed I/O.