Embed Linux in Monitoring and Control Systems

The target vehicle for this project is a vintage intercity transport bus (think Greyhound) whose instrument panel was sparse and mostly nonfunctional. The speedometer cable was twisted off some place back in 40 feet of cable, and the fuel sensor had sunk long ago. What I wanted was an instrument panel more in line with modern practice.

To bridge the gap, I used a laptop computer running the Fedora 20-KDE distribution of Linux as a host, three digital signal processor boards as hardware interface processors (HIPs), a USB/RS422 converter that connects to an RS422 loop linking the HIPs together and some software that I call the Vehicle Monitoring and Control application.

System Architecture

The HIPs are based on a signal processor chip, programmed in C and with no user interface except a heartbeat LED to show that the processor is working to some degree. The HIPs provide signal conditioning circuitry for analog input scaling and optical isolation for control signals plus a few specials like thermocouple converters and a pressure transducer chip. There also are two RS422 receiver/transmitter pairs. One pair connects up-network (toward the host) and the other down-network (toward the other HIPs).

The way this application works is that a message is originated by the host processor and transmitted down-loop to the first HIP. There it may be modified under HIP program control and relayed on down-loop to the next HIP. The last HIP in the "loop" transmits its message up-loop under physical jumper control. Processors closer to the Host simply pass on up what is coming from below in the "loop". The Host is the ultimate receiver of the messages it originates.

A message consists of an SOM byte, an address byte with acknowledge bit, a command byte, four data bytes and two CRC bytes. Going down loop, the HIPs relay a message on a character-by-character basis with a one-character delay per HIP. The addressee of a message sets the acknowledge bit and inserts or extracts data on the fly. So in a short loop like the one here, the host begins receiving the response from the network before it has finished sending the original message. For this loop, the communication rate was selected, arbitrarily, as 57,600 baud, so the loop message time is (9 + 3)/57600 or 208 microseconds. The left portion of Figure 1 depicts the loop topology.

Figure 1. System Architecture

The Vehicle Monitoring and Control (VMC) application will originate messages like "HIP1 set or get register whatever". First, I show how to set up a development environment on a Linux box, and then I talk about how to use the tools made available there to weave together a Linux real-time application that performs the VMC application.

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Rick Brown is a US Navy veteran, holds a BSEE granted in 1970 by the University of Florida, developed atmospheric research instruments as a faculty member of the University of Nevada System, and consulted in the private sector as a developer.