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What is the "RS" in RS232/RS485/RS422?

Let's start out by talking about this nasty "RS" business. So, what the heck does the "RS" stand for? You had better sit down for this one. It means Recommended Standard.  That's right! The RS stands for Recommended Standard. Nothing was really agreed upon or official. At least not in the sense of the "made-by-committee" standards like IEEE-1284 and IEEE-1394.  However, today RS-485 is formally known as TIA/EIA-485, for the associations that oversee the specifications for electronics and telecommunications standards.  Officially the standard is now TIA/EIA-485-A, you may purchase your own copy of the standards here.  To see more interpretation of the standards and further information regarding RS-232/485/422 see our app notes section in this website.

What does this all mean? Because RS standards were merely recommended and technically general, lots of manufacturers develop products that are at best sub-standard. They cut corners and cheat in order to manufacture cheaper products.

We have tried not to make sub-standard products. That is why you don't see any port powered products put out by Integrity Instruments. Engineers have enough headaches the way it is!

Also to reach a better understanding of TIA/EIA/RS-485, read an article on RS-485 written by Bob Perrin for Circuit Cellar Online, July 1999.  Read it here!

Simplex & Duplex

One of the most fundamental concepts of communications technology is the difference between Simplex and Duplex.

Simplex can be viewed as a communications "one-way street". Data only flows in one direction. That is to say, a device can be a receiver or a transmitter exclusively. A simplex device is not a transceiver. A good example of simplex communications is an FM radio station and your car radio. Information flows only in one direction where the radio station is the transmitter and the receiver is your car radio. Simplex is not often used in computer communications because there is no way to verify when or if data is received. However, simplex communications is a very efficient way to distributed vast amounts of information to a large number of receivers. 

Duplex communications overcome the limits of Simplex communications by allowing the devices to act as transceivers. Duplex communication data flows in both directions thereby allowing verification and control of data reception/transmission. Exactly when data flows bi-directionally further defines Duplex communications.

Full Duplex devices can transmit and receive data at the same time. RS232 is a fine example of Full Duplex communications. There are separate transmit and receive signal lines that allow data to flow in both directions simultaneously. RS422 devices also operate Full Duplex.

Half Duplex devices have the dubious honor of allowing both transmission and receiving, but not at the same time. Essentially only one device can transmit at a time while all other half duplex devices receive. Devices operate as transceivers, but not simultaneous transmit and receive. RS485 operates in a half duplex manner

Side-By-Side Specification Chart

Here is the short version of the critical specifications. Unfortunately, these are subject to interpretation by individual manufacturers. That is why RS232 is often regarded as an incredibly non-standard communications protocol.

One important note. You will see that one of the major differences between RS232 and RS422/RS485 is the signaling mode. RS232 is unbalanced while RS422/RS485 is balanced. An unbalanced signal is represented by a single signal wire where a voltage level on that one wire is used to transmit/receive binary 1 and 0: this can be considered a push signal driver. On the other hand, a balanced signal is represented by a pair of wires where a voltage difference is used to transmit/receive binary information: sort of a push-pull signal driver. In short, unbalanced voltage level signal travels slower and shorter than a balanced voltage difference signal.

 

RS232

RS422

RS485

Cabling

single ended

single ended
multi-drop

multi-drop

Number of Devices

1 transmit
1 receive

5 transmitters
10 receivers

32 transmitters
32 receivers

Communication Mode

full duplex

full duplex
half duplex

half duplex

Max. Distance

50 feet at 19.2 Kbps

4000 feet at 100 Kbps

4000 feet at 100 Kbps

Max. Data Rate

19.2 Kbps for 50 feet

10 Mpbs for 50 feet

10 Mpbs for 50 feet

Signaling

unbalanced

balanced

balanced

Mark (data 1)

-5 V min.
-15 V max.

2 V min. (B>A)
6 V max. (B>A)

1.5 V min. (B>A)
5 V max. (B>A)

Space (data 0)

5 V min.
15 V max.

2 V min. (A>B)
6 V max. (A>B)

1.5 V min. (A>B)
5 V max. (A>B)

Input Level Min.

+/- 3 V

0.2 V difference

0.2 V difference

Output Current

500 mA
(Note that the driver ICs normally used in PCs are limited to 10 mA)

150 mA

250 mA

 


DTE & DCE

Let's talk about DCE (Data Communications Equipment) and DTE (Data Terminal Equipment) devices. The difference between DCE and DTE is largely in the Plug and the direction of each pin (input or output). Your desktop PC is termed as a DTE device.

DCE devices use a 25-pin female connector while a DTE device uses a 25 pin male connector. Also, complimentary signals lines like transmit and receive are "swapped" between the two types. Thus, a straight-through cable can be used to connect a DCE device to a DTE device.

DTE

DCE

25 pin male pin-out

25 pin female pin-out

Pin 1 - Shield Ground

Pin 1 - Shield Ground

Pin 2 - Transmitted Data (TD) output

Pin 2 - Transmitted Data (TD) input

Pin 3 - Receive Data (RD) input

Pin 3 - Receive Data (RD) output

Pin 4 - Request To Send (RTS) output

Pin 4 - Request To Send (RTS) input

Pin 5 - Clear To Send (CTS) input

Pin 5 - Clear To Send (CTS) output

Pin 6 - Data Set Ready (DSR) input

Pin 6 - Data Set Ready (DSR) output

Pin 7 - Signal Ground

Pin 7 - Signal Ground

Pin 8 - Carrier Detect (CD) input

Pin 8 - Carrier Detect (CD) output

Pin 20 - Data Terminal Ready (DTR) output

Pin 20 - Data Terminal Ready (DTR) input

Pin 22 - Ring Indicator (RI) input

Pin 22 - Ring Indicator (RI) output

You can effectively convert DCE/DTE devices by using a NULL Modem cable. The null modem cable swaps the complimentary signals and allows a DCE device to act like a DTE and vice-versa.

The following chart depicts the cabling of the DB9 connector found on an IBM-PC type computer.

DTE

9 pin male pin-out

Pin 1 - Carrier Detect (CD) input

Pin 2 - Receive Data (RD) input

Pin 3 - Transmitted Data (TD) output

Pin 4 - Data Terminal Ready (DTR) output

Pin 5 - Signal Ground

Pin 6 - Data Set Ready (DSR) input

Pin 7 - Request To Send (RTS) output

Pin 8 - Clear To Send (CTS) input

Pin 9 - Ring Indicator (RI) input

 

RS485 Multidrop Wiring Diagram

RS485 is sometimes termed as RS485 Multidrop LAN since it can connect several devices in a LAN network environment. These devices are all connected to a single pair wire. Transmit and receive share the same two wires.

Officially the RS485 specification allows only 32 nodes (devices) on the LAN. However, I.C. manufacturers have developed RS485 drivers capable of allowing 128 to 255 nodes on an RS485 LAN. We use these next generation RS485 drivers in our products. This means that you can use our Converters and Remote I/O devices in more expansive situations.

 

RS422 Single Ended Wiring Diagram

RS422 is a "drop-in" replacement for most RS232 applications. It is full-duplex and capable of long distance communications.

RS422 Multidrop Wiring Diagram

In our opinion, RS422 multidrop is a bozo-nono. Quite frankly it is a pain in the posterior because it is a mix of RS485 multidrop and RS422 single ended. In short, you get the worst of both worlds and a wiring headache to boot. There are so many variations to wiring multi-drop RS422, that we do not feel it would do any good to display them here. Let your imagination run wild!