DATA COMMUNICATIONS
© Copyright Brian Brown, 1995-2000. All rights reserved.

Part 8: RS232 Serial Communications

Electrical Interfaces | EIA RS232 | Connectors | Signal Descriptions | Exercise | Transferring Data
Break Out Box | DTE - DTE | Null Modem | RS232D | Summary | Test 4

Introduction
This section introduces the RS232 Serial standard. This is a standard that is used to connect serial devices like Modems to a personal computer. The main features and signals of the standard will be covered.

Objectives
At the end of this section you should be able to

• state what an electrical interface is
• list the major pin numbers and signal functions for RS232
• distinguish a male and female RS232 connector from other connector types
• describe the use of a breakout box
• design a null modem cable

Electrical Interfaces
An electrical interface is the connection between two devices. There are two common data interfaces that specify international standards for low speed data communication.

• CCITT V.24
  Does not specify pin numbers. Details the types of signals to be exchanged. Specifies the absolute voltage levels, impedance's, and timing characteristics for each line.
• EIA RS232-C
  Uses V.24 recommendations and further specifies the mechanical connector type and pin numbers to be used (DB-25 pin connector).

  THE EIA RS232-C STANDARD
Specifies a 25 pin connector as the standard interface in data communication networks, with lettering pin designations for ground, data, control and timing circuits. The table below shows the designations for each of the 25 pins of the standard.

This diagram shows an RS232 female connector, with the designated signals shown for each pin.

DATA COMMUNICATIONS EQUIPMENT (DCE)
An example of a DCE is a modem. A DCE is fitted with a 25 pin female connector.

DATA TERMINAL EQUIPMENT (DTE)
An example of a DTE is a computer terminal. A DTE is fitted with a 25 pin male connector.

There are two main parts, a mechanical and an electrical standard.

• EIA RS232-C Mechanical Standard
  Female connector is connected to DCE and male connector to DTE. Short cables of less than 15 meters (50 feet) are recommended. The pin assignments detailed above must be used.
• EIA RS232-C Electrical Standard
  All circuits carry bi-polar low-voltage signals, measured at the connector with respect to signal ground (AB), and may not exceed ±25 volts. Signals are valid in the range ±3 volts to ±25 volts. Signals within the range -3 volts to +3 volts are considered invalid.

  For data lines, binary 1 (a high) is represented by -3 volts to -25 volts, whilst binary 0 is +3 volts to +25 volts.

  For control lines, OFF is represented by -3 volts to -25 volts, whilst binary 0 is +3 volts to +25 volts.

RS232 Connectors
The following diagram shows the 25-pin connector used for the DTE interface. It is a MALE connector, which has 25 pins. Beneath it is the 25 pin FEMALE connector used on the DCE interface. Note that the connectors have a longer side at the top and a shorter side at the bottom. This is to prevent the plugs being inserted into the connectors the wrong way round.

RS-232 Signal Descriptions
The interface transfers data between the computer and the modem via the TD and RD lines. The other signals are essentially used for FLOW CONTROL, in that they either grant or deny requests for the transfer of information between a DTE and a DCE. Data cannot be transferred unless the appropriate flow control line are first asserted.

The interface can send data either way( DTE to DCE, or, DCE to DTE) independently at the same time. This is called FULL DUPLEX operation. Some systems may utilize software codes so that information may only be transmitted in one direction at a time ( HALF DUPLEX), and requires software codes to switch from one direction to another (i.e., from a transmit to receive state).

The following is a list of common RS232 signals.

• Request To Send (RTS) This signal line is asserted by the computer (DTE) to inform the modem (DCE) that it wants to transmit data. If the modem decides this is okay, it will assert the CTS line. Typically, once the computer asserts RTS, it will wait for the modem to assert CTS. When CTS is asserted by the modem, the computer will begin to transmit data.
  
• Clear To Send (CTS) Asserted by the modem after receiving a RTS signal, indicating that the computer can now transmit.
  
• Data Terminal Ready (DTR) This signal line is asserted by the computer, and informs the modem that the computer is ready to receive data.
  
• Data Set Ready (DSR) This signal line is asserted by the modem in response to a DTR signal from the computer. The computer will monitor the state of this line after asserting DTR to detect if the modem is turned on.
  
• Receive Signal Line Detect (RSLD) This control line is asserted by the modem, informing the computer that it has established a physical connection to another modem. It is sometimes known as Carrier Detect (CD). It would be pointless a computer transmitting information to a modem if this signal line was not asserted. If the physical connection is broken, this signal line will change state.
  
• Transmit Data (TD) is the line where the data is transmitted, a bit at a time.
  
• Receive Data (RD) is the line where data is received, a bit at a time.

A lot of signals work in pairs. Some signals are generated by the DTE, and some signals are generated by the DCE. If you were measuring the signals on a computer which was NOT connected to a modem, you could only expect to see those signals that the DTE can generate.

Something to think about
You have connected an external modem to a computer using an RS232 cable. After loading the application software, it reports "Modem is not turned on". You check, and find the modem is turned on. Gaining access to a multi-meter device, which is used to read the state of pins on the RS232 connection, which pin do you think you should check to verify that the modem is turned on?.

How to exchange information between a DCE and DTE
Now, lets look at the sequence that occurs when data is transferred between a DTE and a DCE. The data can only be transferred after the correct sequence of signals is followed, for instance, there is no point sending data if the modem is turned off. Lets go through each of the steps involved (i.e., signal line assertions required) to transmit and receive characters across the RS232 interface.

TRANSMITTING DATA (DTE to DCE)

• 1: Assert DTR and RTS
• 2: Wait for DSR
• 3: Wait for CTS
• 4: Transmit the data

Step 1 and 2 are essential to ensure that the modem is on-line and connected to another modem. Waiting for DSR checks that the modem is on-line.

RECEIVING DATA (DCE to DTE)

• 1: Assert DTR
• 2: Wait for DSR
• 3: Receive the data

Break-out Boxes
An RS232 breakout box is a device that allows you to monitor the RS232 connection, and connect various signal lines together. It is placed between the DTE and the DCE, so you can see the state of the various signal lines and perform interconnection if required.

Using the breakout box is a matter of determining the signals being asserted and performing interconnection of signal lines if required. LED's are used to indicate the state of the signal line. RED indicates an active signal (high), and GREEN an inactive signal (low).

CONNECTING TWO DTE DEVICES TOGETHER
Often, two DTE devices need to be connected together using a serial link. This is for file transfer or printer access. The problem is that DTE devices expect to talk directly to DCE devices, not another device of the same type. DTE's cannot generate signals like DSR and CTS, so connecting two DTE's together will result in neither getting permission to send, and thinking that the modem is off-line (by not receiving DSR).

To allow the interconnection of two DTE devices without using DCE's, a special type of cable must be used. This is called a Null Modem Cable, which fools the DTE into thinking that it is connected to a DCE device. In this case, modems are not used, so the connection looks like.

DESIGNING A NULL-MODEM CABLE
In designing a NULL-MODEM cable, the DTE signals from one computer are swapped over as inputs to supply the DCE expected signals on the other DTE.

As you can see from the diagram, when two DTE's are connected together, the signal lines from one DTE are transposed to the other DTE, fooling it into thinking that it is communicating with a DCE.

RS232D (9 pin Connector)
The following table illustrates the 9 pin serial connector as found on most PC's today. This has all but replaced the previous 25 pin connector found on earlier PC's.

Summary
The RS232-C interface connects a DTE (computer) to a DCE (modem). It is a 25 pin interface, where some signals are generated by the DTE, and some by the DCE. It supports slow speed data communications. The DTE uses a male connector and the DCE a female connector.

A breakout box is a device used to monitor or change the state of the interface lines. It is handy in troubleshooting problems. A null modem cable transposes signals so that a DTE can exchange data with another DTE.

Test 4 [JavaScript]
Now it is time to review this section and discover how much you have absorbed. Click on the hyperlink above to begin the test.

© Copyright B Brown. 1995-2000. All rights reserved.