• Table of Contents

9.3 The RS-232 Standard

<aside> <img src="/icons/map-pin_gray.svg" alt="/icons/map-pin_gray.svg" width="40px" /> The need to exchange data between the MCU (Marvel Cinematic Universe) microcontroller unit and peripheral devices can be satisfied by using parallel data transfer. However, there are drawbacks, which the serial communication interface (SCI) solves.

We have used this interface before to communicate with and download programs onto the demo board for execution during labs.

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So what exactly is the RS-232? It stands for “Recommended Standard 232,” which in layman terms, is the standard that sets out to ensure compatibility between the host and peripheral systems.

Figure 9.1 A data communication system.

In data communication terms:

• The host system, such as computers and terminals are called data terminal equipment (DTE).
• The peripheral system, such as modems, bridges, and routers are referred to as data communication equipment (DCE).

TIA-232 Specifications

Though it has now since been revised and updated, as the TIA-232, to meet the evolving needs of serial communication applications. Those standards can be broken down into four aspects:

<aside> <img src="/icons/map-pin_gray.svg" alt="/icons/map-pin_gray.svg" width="40px" /> This will be a condensed and quick summary of each specification — highlighting the important parts of each one. Refer to the textbook, for an in-depth coverage of each one.

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1. Electrical specifications: Below are the electrical specifications which are of interest to us.

   • Data rates of up to 20,000 bits per second.
   • Voltages of $-3\ \textrm{V}$ to $-25\ \textrm{V}$ are considered logic 1.
   • Voltages of $+3\ \textrm{V}$ to $+25\ \textrm{V}$ are considered logic 0.
   • Transfer distance should be within 15 m.

2. Functional specifications: The TIA-232E standard specifies 22 signals, divided into six categories.

   • Signal ground and shield.
   • Primary communications channel.
   • Secondary communications channel.
   • Modem status and control signals.
   • Transmitter and receiver timing signals.
   • Channel test signals.

   

   Figure 9.2 Functions of EIA-232E signals.

3. Mechanical specifications: The TIA-232E uses a 25-pin D-type connector, or a subset which is a 9-pin connector (DB9).

   • A 25-pin D-type connectors is specified.

   

   Figure 9.3a EIA-232E DB25 connector and pin assignment.

   • Since only a small subset is actually used, a 9-pin connector (DB9) is used in most PCs.

     

     Figure 9.3b EIA-232 DB9 connector and signal assignment.

4. Procedural specifications: The sequence of events that occurs during data transmission using the TIA-232 standard.

   • Common examples include connecting two DTEs via DCEs. Below shows two DTEs connected via a direct link.

   

   Figure 9.5a Point-to-point asynchronous connection**.**

   • The sequence of events is displayed below.

   

   Figure 9.5b Sequence of events occurred during data transmission over direct link.

Data Format

The SCI uses asynchronous data transfer, where data is transferred character by character, and start and stop bits are used to indicate the beginning bit.

Figure 9.6 The format of a character.

This method of transmission is used when data (characters) are sent intermittently as opposed to in a solid stream. A character consists of a start bit (0), 8 to 9 data bits, and a 1 to 2 stop bits (1). It should be noted that the LSB is transmitted first and the MSB is transmitted last.