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Part 5: RS232 Serial Communications
Introduction
This section introduces the RS232 Serial standards. 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.
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
• CCITTV.
Does not specify pin number. 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.
INTERCHANGE CIRCUIT
NO.
PIN
NO
DESCRIPTION
AA 101 1 Protective Ground BA 102 2 Transmit Data BB 104 3 Receive Data CA 105 4 Request To Send CB 106 5 Clear To Send CC 107 6 Data Set Ready AB 102 7 Signal Ground CF 109 8 Receive Line Signal Detect/Carrier Detect -- -- 9 Reserved -- -- 10 Reserved -- -- 11 Unassigned SCF 122 12 Secondary RLSC SCB 121 13 Secondary CTS SBA 118 14 Secondary TD DB 114 15 Transmitter Signal Element Timing SBB 119 16 Secondary Rd DD 115 17 Receiver Signal Timing Element -- -- 18 Unassigned SCA 120 19 Secondary RTS CD 108.2 20 Data Terminal Ready CG 110 21 Signal Quality Detector CE 125 22 Ring Indicator CH/CI 111/112 23 Data Signal Rate Selector DA 113 24 Transmit Signal Element Timing -- -- 25 Unassigned
There are two main parts, a mechanical and an electrical standard.
- EIA RS232-C Mechanical Standard
Female connector is connector is connected to DCE (DATA COMMUNICATION EQUIPMENT An example of a DCE is a modem.) and male connector to DTE. ( DATA TERMINAL EQUIPMENT An example of a DTE is a computer terminal.). 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
- 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 something 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 NO/t connected to a modem, you could not expect to see those signals that the DTE can generate.
The following table lists some of the signal pairs And the device responsible for generating them.
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 pint 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.
DTE DCE
TD RD
RTS CTS
DTR DSR
TRANSMITTING DATA (DTE TO DCE)
- 1: Assert DTR and RTS
- 1: 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
All RS232 breakout box is a device that allows you to monitor the RS@#@ connection, and connect various signal lines together. It is placed between the DTA 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 devices 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 is connected to a DCE device. In this case, modems are not used, so the connection looks like.
