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13

Simple RS485 Network with Microcontrollers

13.1 In this Chapter

This chapter contains the description of a device capable of reading the status of a number of digital inputs, or the values of a number of analog inputs, to store this information, and to report it when it receives a specific interrogation from a master device. It is implemented using an AVR microcontroller, and uses a RS485 line to communicate with the master. The aim of this chapter is to introduce the basic concepts of distributed data acquisition systems.

13.2 The Hardware

The simple data acquisition modules described in this chapter are meant to be connected as slaves in a RS485 network, as shown in Fig. 13.1.

The master device in this network is a personal computer (PC), which uses one of the asynchronous communication ports (COM1/COM2) connected to the network through a RS232-to-RS485 interface converter.

Two different slave devices are described for this application: one, called SLD, is designed to read and report the status of three digital inputs, such as relay contacts; the other, called SLA, reads the values of three analog inputs in the range 0–2.56 V.

It is possible, in principle, to design and add more slave devices, with different functions, provided that they communicate according to the same protocol as the

Fig. 13.1. Block diagram of the network described in this chapter

164 13 Simple RS485 Network with Microcontrollers

others. According to the recommendations of the RS485 standard, the total number of slaves connected to the same bus is limited to 32.

13.2.1 The RS232-to-RS485 Converter

This circuit converts the ±12 V voltage levels required by the RS232 interface of the PC used as a master device in the network, to differential signals compatible with the RS485 communication bus. The schematic of the circuit is presented in Fig. 13.2.

The actual RS485 interface circuit is IC3 –SN75176, described in Chap. 3. This converts TTL levels to differential signals, and its line driver can be controlled to enter a high-impedance status by means of the input DE (Driver Enable, active HIGH). This allows the implementation of a two-wire, half-duplex differential communication bus.

IC4 –MAX232 converts TTL to RS232 voltage levels and vice-versa. The inverting gate IC2A (74HC04) is used to provide proper polarity for DE, so that the interface signal DTR (Data Terminal Ready), available on pin 4 of the nine-pin interface connector, can be used to control the direction of data transfer on the RS485 bus.

The external power supply VPP = +12 V is applied on connector X1. VPP is reduced to +5 V by means of the voltage regulator IC1 –LM7805, to supply local circuits, but is also made available on the connector X2, along with the differential data lines, in order to provide power to the slave devices. For this particular application VPP must be able to deliver at least 200 mA.

Note the resistors R1, R2, connected to VCC and GND. Their purpose is to maintain the voltage levels on the data lines A and B to a steady potential when all the line drivers of the devices connected to the RS485 bus are in the high-impedance status.

VCC

Fig. 13.2. Schematic of the RS232-to-RS485 converter

Fig. 13.3. PCB layout for the RS232-to-RS485 converter

Figure 13.3 shows a possible layout for the printed circuit board for this circuit.

13.2.2 The Digital Input Module

This circuit shown in Fig. 13.4 is one of the simplest microcontroller structures possible.

The circuit comprises the microcontroller AVR AT90S2313, with very few external components: the RS485 interface circuit IC3 (SN75176), the voltage regulator IC1 (LM7805), and the oscillator and RESET circuits.

The digital inputs are read on the PB0–PB2 I/O lines, which must be software configured to use the internal pull-up resistors, and another I/O line, PD2, enables the line drivers of the interface circuit SN75176.

Fig. 13.4. Schematic of the SLD module

GND

Fig. 13.6. Adding the RS485 interface circuit to the AVR development board

Table 13.1. Connections between the RS485 interface circuit and the MCU

The purpose of the modification is to add a SN75176 circuit, which implements the RS485 interface, and the X2 connector in the custom area of the development board. These components must be connected to the MCU according to Table 13.1.

With the RS485 interface, the development board is capable of emulating all the functions of the SLD and SLA modules described above.

13.3 The Software

The principles of a simple master–slave network protocol have been described in Chap. 3. The following paragraphs contain an example of the implementation of a simple microcontroller network based on these principles.

13.3.1 Description of the Communication Protocol

The devices connected to the RS485 bus communicate according to the following set of rules:

•Communication data consists of fixed-length packets, having a predetermined structure.

•All the information in the data packets is ASCII encoded.

•All data transfers are initiated by the master device, which sends interrogation or command packets to specific slave devices.

•All slave devices are identified by a unique address.