Half, full and Simplex|Popular Computing Weekly)
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David Wallin explains duplex and takes a look at the RS232 interface

This week I'll be looking at Ihe RS232 interface. But first a full explanation of duplex is required.

For full information on the use of each pin of the RS 232c. see Hotline- a persona! guide to computer communcations by Ben Knox. (Century Communications, £6.95. ISBN 0-09-160931-3). This book was an information source for this week's article.


Transmission and reception of data may be in one of three ‘duplexes' or modes, they are: Full Duplex. Half Duplex and Simplex.

Simplex-This mode of operation allows data to either be sent or received, but not both. Simplex Data Transmission means that data may be transmitted, but not received. Similarly, Simplex Data Reception means that data may be received, but not transmitted. A normal radio receiver is a received, but not transmitted. A normal radio receiver is a simlex device, you pick up sound transmitted from a studio, but you can't talk back.

Half Duplex - Half duplex allows data to be sent or received but not at the same time. When transmitting data, you cannot receive anything, and when receiving, you can't transmit. ‘Walkie-Talkies' and CBs are half duplex devices. On a computer-modern link the TxD and RxD pin are required. Also the protective ground should be wired up.

Full Duplex - Full duplex allows you to receive and transmit at the same time. A telephone is a full duplex device, you can hear and talk at the same time. Most home micros will work with full duplex modems, but are not capable of doing two things at once, (ie, transmitting and receiving) and so they operate in half duplex. Micros like the Amiga which can multi-task would be able to receive and transmit at one time. Full duplex is expensive and usually requires four lines. TxD. RxD. protective ground and signal ground (pin 9).

The most common duplex is half and. unless stated otherwise, the information in this article applies to half duplex operation.

How RS 232 began

Over 20 years ago, the Electronics Industries Association of Washington DC. decided that they needed a standard for connecting any computer (be it micro, mini or mainframe) to any serial device (ie modem, printer or robot). A standard was set and this was known as ‘Agreement RS232'. The most common RS standard used today, the V24 version, is agreement RS 232c which was introduced in 1969

Many home micros (including ttie BBC) have an RS 423 interface port. This is a budget version of the full 232c. As most data links (connection of micro and device) require usse of only five of the 25 pints, the RS 423 was designed as a five pin serial port Also many micros that claim to have an RS 232 or an RS 232c. in fact have a cut down version; very few use all of the 25 pins.

The RS 423

As mentioned above RS 423 has only five pins. This is because most of the other pins are not required, and those that are can be 'created' by combining the available pins. The five pins used are:

Name Abbreviation

Pin number

Protective ground (Gnd) 1
Transmit data (TxD) 2
Received data (RxD) 3
Request to send (RTS) 4
Clear to send (CTS) 5

Some modems have the inputs and outputs (TxD and RxD) labelled differently, so check the manual for proper wiring up details. In fact, only three pins are essential as the CTS and RTS lines are hardly ever used these days. This is because their job can be done by a buffer, an area of Ram (about 128 bytes. (£K) in which data waits as if in a kind of queue.


The purpose of CTS and RTS are to prevent data collisions. This is when data Is going in both directions, on a half duplex line and there is literally a crash as the two collide. This results in loss of and damage to data. To prevent this CTS and RTS are available.

If the computer wishes to send data, it raises or take high the RTS line (it requests permission from the modem to send some data). In plain English, it applies to voltage to It. When the modem is ready and able to send data, it raises the CTS line (telling the computer it is clear to send) The data is then sent. After it has all been sent. RTS and CTS are lowered, the voltage is dropped.

Full duplex has no need for the CTS and RTS as Txed (transmitted) data and Rxed (received) data travel along different wires.

RS 232c link

It is useful to know what all the pins on an RS 232c are for, so here's a list Pin

No. Description Abbreviation
1 Protective ground Gnd
2 Transmitted data TxD
3 Received data RxD
4 Request to send RTS
5 Clear to Send CTS
6 Data set ready DSR
7 Signal ground Gnd
8 Received line signal detector  
9 Reserve  
10 Reserve  
11 Unassigned  
12 Secondary received line signal deleclor  
13 Secondary clear to send 2nd CTS
14 Secondary transmitted data 2nd TxD
15 Transmitter signal element timing DCE source
16 Secondary received data 2nd RxD
17 Receiver signal element timing DCE source
18 Unassigned  
19 Secondary recuest la send 2nd RTS
20 Data terminal read DTR
21 Signal quality detector  
22 Ring indicator RI
23 Data signal rate selector  
24 Transmit signal element timing DTE source
25 Unassigned  

Unassigned means that the pin has no use. the two unassigned pins take the number of pins up to 25. a standard connector size Reserved means that the pin is there for the manufacturers to do what they like with. They are not subject to either RS 232 or CCITTT regulations and may be used for testing, etc. They usually perform functions which only that device has.

The connectors

The two common connectors are:

  1. The standard 25 way RS 232 D' socket An elongated D' shape. The female is usually found on the computer and the male on the modem/device The pin numbers correspond to the RS 232 pm number
  2. The five pin RS 423, as found on the BBC and many other cheaper home micros. The computer usually has the female connector.

David Wallin , Popular Computing Weekly

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