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October 2009 Doc ID 16144 Rev 1 1/28
AN3031
Application note
SCLT3-8 input termination demonstration board
STEVAL-IFP007V1
1 Introduction
The SCLT3-8BT8 is an eight channel termination used for interfacing automation digital
inputs. It is designed for 24 VDC applications and meets type 3 input characteristics in
accordance with IEC 61131-2 (programmable controllers international standard). The chip
integrates an SPI peripheral output for communication with logic/ASIC/microcontroller. Its
structure allows the transfer not only of the data but also of additional information including
thermal alarm, undervoltage indication and parity bits. It is particularly beneficial for
applications with high channel counts. The serial communication allows for reduction of the
number of lines, which in most applications need to be galvanically isolated.
The device optimizes the input VI characteristic shape to reduce power dissipation. This
becomes critical when the application enclosure requires higher protection levels, such as
IP codes IP65 and IP67. These closed cabinets are used in various automated lines,
including the food industry. The power dissipated inside the module is very limited, as there
is usually no air circulation inside the box. The current limitation also reduces power losses
on the sensor side.
Another important parameter is electrical protection and susceptibility of the application.
The SCLT3-8BT8 implements large protection diodes on each input and supply voltage line.
Serial resistors ensure a higher level of susceptibility than required.
Thanks to its compact package, an SCLT3-8BT8 application occupies little space on the
PCB compared to discrete solutions.
Figure 1. STEVAL-IFP007V1 demonstration board
www.st.com
