CS452 - Real-Time Programming - Fall 2009

Lecture 3 - Communications: RS-232

Reminder

Practical Details: pdf
Assignment deadlines
How to compile and run your first program: pdf

Assignment 1

Overall System

Provided and maintained by CSCF

Linux systems
- cross compiler: runs on 86_64, produces code for ARM
- GNU toolchain: compiler, assembler, link editor
  - You will notice that my makefile separates
    - compilation to assembly code,
    - assembling the assembly code, and
    - link editing.
- need to login explicitly to linux.student.cs
TFTP servers
- need to type IP number explicitly

TS-7200

`COM' ports

Connected to UARTs

RS-232

Only really two

Ethernet port

Reset switch

Timers

EP-9302

System on chip

ARM 920T core, implementing ARM v4T instruction set
Two co-processors
1. System controller, MMU
2. Maverick Crunch floating point unit
Peripherals
1. UARTs
2. Timers
3. DIO
4. A/D
5. etc.

Memory

Byte addressable, word size 32 bits

32 Mbytes of RAM, starting at 0x00000000
4 Mbytes of flash RAM, starting at 0x60000000
- Contains RedBoot, which is loaded into RAM at startup
Special locations at low addresses
Special locations above 0x80000000

Separate instruction and data caches

Software

Compiler

GNU tool chain

when you are getting started optimizing is usually a bad idea
software multiplication, division, floating point from libgcc.a
Makefile
target.ld

RedBoot

Partial implementation

fconfig :: NOT
load (tftp)
examine, copy, fill memory

Returns when program terminates

Busy-wait IO

COM2 uses monitor; COM1 goes to train

initialization
output
input

Data Communication using RS-232

Communication protocol existing since forever.

description of a wire, and the signals on it.

Why do we still use it in the age of USB, etc.

simple
inexpensive
robust
easy to implement

Very simple RS-232

Three wires

ground
transmit
receive

Wired like this

GND ---------- GND

XMIT --\ /---- XMIT
        X
RCV----/ \---- RCV

What could be simpler?

BUT, ...
This is actually called a null modem.
To see why we need to look at the pins on the connector.

Less Simple RS-232

Distinguish Data Terminal Equipment (DTE) from Data Circuit-terminating Equipment (DCE)

On the DE-9 connector we have for DTE

5 -- GND
3 -- XMIT
2 -- RCV

and for DCE

5 -- GND
3 -- RCV
2 -- XMIT

A very simple `straight through' cable connects these perfectly. But when we conect DTE to DTE we need the cable above, which is called a `null modem'.

Train RS-232

Three wire works fine as long as both ends of the wire are much faster than the wire itself. But the train controller is very slow. Why?

Wait for mechanical sensors.
High electrical noise

The train controller needs to be able to say STOP. How is this done?

Software Flow Control

XOFF, XON

Hardware Flow Control

CTS, RTS

DTR, DSRWhat Happens if You Omit Flow Control

Return to: