RS232
Protocol
INTRODUCTION
This page has been written to clarify the serial protocol used in RS232. Using this,
one should be able to develop a RS232 serial port in VHDL. Source code is for
a predeveloped RS232 serial port is included in this document.
BACKGROUND
RS232 is a basic serial protocol avalible to most hardware devices. In the most basic
case, the RS232 serial protocol consists of two lines: data rx line and data tx line. What this
implies is that both ends of the RS232 line must be in sync since to external clock is
syncronizing the tranmission of data. Othere signals can be added also for flow control such
as the Clear To Send signal and Clear To Recieve signals. Both are Active low.
Serial Protocol
The Serial Line remains high until a data is being sent. The number of data bits can vary
from 5 to 8 data bits; however, 8 bits is the general standard. The data bits can include
a parity bit for error control. Parity will not be disscussed in detail here, please look under references
for more information on error control. Data transmission begins with a start bit, the data payload, followed
by a stop bit. The stop bit can vary also, from 1, 1.5, to 2 bits long. NOTE: the code provided with
this app note needs a 2 bit long stop bit. The start bit must be the opposite level of the default level
to indicate that data is being sent, thus start bits are low. Stop bits are high, and the bits in between varies
with the data being sent. The least significant bit is sent first followed by the most signicant bit. Figure 1
illustrates a typical data load.
Figure 1: Serial Transmission Line with Data Payload 01011001
Code
Note that the following code has been taken from the previous App Note RS232
Serial Port. However, there were unexpected bugs in the code, thus it had to be
modified by Ben Talbot, Timmy Li, and Andrew Ling.
The file serialPort.vhd uses five files that will need to be in your working directory. The files myShiftOut.vhd and myShiftRight.vhd were created automatically by the wizard in MaxPlus2 to optimize the lpm_shiftreg, and are used by RS_232_Out and RS_232_In respectively.
RS_232_Out.vhd
RS_232_In.vhd
myShiftRight.vhd
myShiftOut.vhd
clkdiv.vhd
For more information please contact Andrew Ling.
Group Members:
Darren Gonek
Guillermo Barreiro
Andrew Ling
Shyam Chadha
Timmy Li
Reid Orsten
Group AppNotes
TTL
to Serial
DC
Motor Control
CodeWarrior
Tutorial
Surviving VI
RS232 Protocol
RS232
Software Support
References:
Maxim Semiconductor (Free samples of
MAX232A IC)
RS232
Serial Port
http://www.oreilly.com/reference/dictionary/terms/P/Parity.htm
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