Electronic Design

A Network Of RS-485 Smart Temperature Sensors

The system described here implements a smart temperature-sensor network. In this particular application, each of the intelligent network nodes is equipped with two temperature sensors.

The network consists of a two-wire twisted-pair cable and multiple RS-485 transceivers (Fig. 1). With this infrastructure, it’s possible to communicate reliably using line lengths up to 4000 feet (1.2 km) without repeaters over a network containing more than 100 nodes. This design has been successfully implemented for 18 nodes (36 temperature sensors).

By using this architecture, the system can monitor the temperature of up to 36 points of a building to efficiently regulate the heating system. The system also can acquire and store the temperature profile of different points, allowing the temperature variations to be analyzed.

A standard PC acts as the network master and the temperature-sensor boards act as slaves. To interface the PC’s RS-232 serial port to the RS-485 network, a simple converter has been designed (Fig. 2). The speed and word length chosen in this development are 9600,N,8,1. Selection of higher speeds would reduce the transmission distances.

The block diagram of the temperature-sensor cards is shown in Figure 3. The complete schematic of the PIC12C508-based temperature-sensor card is shown in Figure 4. After power-on or reset, the PIC12C508 checks the board ID number and starts checking the messages circulating along the network. If any messages have the matching board address (ID), the microcontroller executes the command and sends an answer. The format of the messages exchanged between master and the slaves is shown in Figure 5.

If the processor detects a system or hardware error (i.e., no MAX1617 connected, a checksum error, or an end-offrame byte error), the error indicator LED (D1) is turned on until a correct message is received.

When the temperature of the sensors is out of range, an LED (D2) is switched on. The temperature warning limits are programmable.

The address is selected with the three on-board jumpers that change the state of three pins (ADD0 and ADD1 of MAX1617, and GP5 of PIC12C508 through a 10k resistor). The different states of these pins and the corresponding addresses are shown in Table 1. The different commands that have been implemented are defined in Table 2.

The twisted-pair cable must connect J1 of the RS-232/RS-485 converter to J1 of the temperature-sensor boards—pin 1 with pin 1 and pin 2 with pin 2. The JP1 jumpers on both types of boards have to be closed for the two boards at the end of the network for proper termination.

With a few changes, the system can be electrically isolated to provide more noise immunity in industrial applications. For these situations, it’s also important to use a shielded cable connected to ground in the converter board, as well as to ground through a series capacitor in the rest of the boards.

With some other changes, it’s possible to connect the sensor boards to a modem and have the master PC located many kilometers away.

The program listings for the smart temperature-sensor element and the PC master control can be found on the Ideas For Design section of the ELECTRONIC DESIGN web site at: http://www.elecdesign.com.

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