Servo - July 2009

Implementing a Low Speed, Low Cost

Communications Protocol

by Fred Eady

(that you’ve probably never heard of) — Part 2

The word “vehicle” is used at least eight times within the LIN Specification document. I searched the LIN document for the word “robot” and received zero hits. Thus, it’s rather obvious that the LIN Specification is definitely aimed at use in automobiles. Fortunately, the LIN protocol is a very well thought out and its disciplines lend themselves well to non-automotive robotic things. LIN is not designed to turn motor shafts, service sensors, or make decisions. LIN is a communications conduit that carries the commands to turn motor shafts and transports the data gathered from sensors. Our goal is to assemble a set of software routines that will take advantage of the LIN protocol. How you put those routines to use is up to you.

The LIN Hardware

All of the master and slave LIN functions we create will be based on the hardware design depicted in Schematic 1. The LIN node hardware grinning at us in Photo 1 can be configured as a LIN master (master configuration jumper present at JP1) or LIN slave node.

A Microchip LIN Serial Analyzer will be used to complement our LIN node hardware. The LIN Serial Analyzer shown in Photo 2 can also be configured as a master or slave node. The LIN Serial Analyzer will be used

PHOTO 1. This LIN hardware is battle tested. We’ve already used this hardware to successfully transmit a LIN frame. Note that the master configuration jumper (JP1) is not positioned in the master mode position which configures this node as an official LIN slave.

to assist us in writing our LIN drivers as it can be programmed to send master task messages as a master node or respond to master task messages as a slave node.

LIN 101

A typical LIN bus supports a single master node and multiple slave nodes. The collection of a LIN master and slaves along with the physical bus wire is called a LIN cluster. LIN nodes are based on hardware equipped with a UART interface that supports standard RS-232 principles

PHOTO 2. The LIN Serial Analyzer is supported by a Windows application which enables LIN message logging, master mode messaging, and slave mode response. You can develop your LIN-based applications without this tool, but it sure came in handy while writing the code for this article.