I2C - RS-485 adapter

Jaakko Ala-Paavola
September 29th, 2001

Status: Not implemented

Date History
2001/09/28 Initial draft
2001/09/29 Added more PCBs

Table of contents

Introduction

The motivation of this project is to extend the widely supported I2C bus for longer distances to apply as a home control network. I2C specification [1] implements simple 2-wire data communication protocol suitable for communication between sensors and microcontrollers. Philips introduced the bus at early 70s, but it's still very usefull even today. Thank's to the old-age there are thousands of devices having I2C interface built-in. Practically all modern PC motherboards includes I2C bus for health status gathering and most operating systems have support for it, including Linux [2].

The drawback of the bus is it's electrical characteristics that allows bus length of only few meters. Originally the bus was designed for device internal communication, inter-ic as the name says, thus few meters was considered sufficient. To apply I2C protocol in house-wide control network, one have to convert electrical signal into form that better suits for longer distances. Multinode networks based on twisted pair differential signals are very good for the purpose. EIA/RS-485 [3] specifies such a network with maximum length of 1000 meters and 32 receivers and transmitters connected. With bridge-circuit one may multiply the number of nodes.

This document introduces a simple bi-directional I2C - RS-485 adapter.

Schematic diagram

Both buses are half-dublex bi-directional, thus the adapter must be bi-directional. In order to prevent signal loops the circuit has mutual exclusion in signal switching. Signal from one direction disables signals going to another direction.

I2C bus is active low ans RS-485 has active high, thus signal levels are inverted. This is done by NOR-gates. First NOR-port (IC2/1) is considered as an inverter with enable signal. Input signals of second NOR-port (IC2/2) are connected together, making the port work as a normal inverter. TTL-RS485 conversion is made by industry standard RS-485 tranceiver, there are plenty of manufacturers offering interchangeable parts [4].

I2C bus has two signals: data and clock, thus the circuit in figure 1 must be dublicated in the actual device.

schematic diagram

Figure 1. One half of the circuit.

Components

Printed Circuit Board

The ensure reusability of design, the device is designed as a module, connected with a pin header connector to main application. There are three different PCBs presented here, all having the very same functionality.

PCB PCB PCB

Figure 2. Printed circuit board

Components

Table 1. Module interface specification
Pin Function Pin Function
1, 2 GND 11, 12 I2C 1
3, 4 485 2-A 13, 14 Vcc
5, 6 485 2-B 15, 16 485 1-A
7, 8 Vcc 17, 18 485 1-B
9, 10 I2C 2 19, 20 GND

Pin order in the interface is mirror symmetric with both axis. The module can be connected which ever way.

solder side solder side solder side

Figure 3. Solder side

component side component side component side

Figure 4. Component side

The schematic diagram and PCB are drawn with Eagle editor, made by German CadSoft [5].

References

1. I2C bus specification

2. I2C driver project for Linux 3. EIA/RS-485 Specification 4. MAX485 Datasheet 5. Eagle PCB & Schematic editor Up to higher level