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

Introduction

The motivati= on 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 communica= tion between sensors and microcontrollers. Philips introduced the bus at early 7= 0s, 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 b= us length of only few meters. Originally the bus was designed for device inter= nal 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. <= /p>

Schematic diagram

Both buses are half-dublex bi-directional, thus the adapter must be bi-directional. In ord= er to prevent signal loops the circuit has mutual exclusion in signal switchin= g. 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. Fi= rst NOR-port (IC2/1) is considered as an inverter with enable signal. Input sig= nals 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 tranceiv= er, there are plenty of manufacturers offering interchangeable parts [4]= .

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

3D"schematic

Figure 1. One half of the circuit.

Components

IC1  MAX485 / LTC485 / D=
S485 / SP485 / ADM485 / SN75167B / etc

  &nb=
sp;    RS485 tranceiver

IC2  74LS33 Quad-NOR, Open-Collector

R1   =
2,2 kOhm resistor

R2   =
220 Ohm resistor

Printed Circuit Board

The ensure reusability of design, the d= evice 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.

3DPCB