RS-232 cable Wiring & Testing

COM Port (OR) RS-232 cable Wiring & Testing

As A technical background eSecurity Professional, many time got call “my Access Controller communication has RS232 enable How we connect with Computer (COM Port), is there any layout” Sometime “Successfully testing via my Laptop but Customer computer not responding, any distance or new programming is there”. I remember in year 2006 me also facing this type of problem with an Access Controller; I would be like to share the myth.

Com Port (Com1 / Com2 etc)= Serial Port = RS232 = Consol.

The wiring of RS232 has always been a problem. Originally the standard was defined for DTE (data terminal equipment) to DCE (data communication equipment connection), but soon people started to use the communication interface to connect two DTEs directly using null modem cables. No standard was defined for null modem connections with RS232 and not long after their introduction, several different wiring schemes became common. With Digital Equipment Corporation tried to define their own standard for serial interconnection of computer devices with modified modular jack connectors. This interfacing standard became available on most of their hardware, but it wasn't adopted by other computer manufacturers. Maybe because DEC used an non-standard version of the modular jack.

Very interesting is the RS232 to RJ45 wiring standard proposed by Dave Yost in 1987, based on earlier wiring schemes used at Berkeley University. He tried to define a standard comparable to DEC, where both DTEs and DCEs could be connected with one cable type. This standard was published in the Unix System Administration Handbook in 1994, and has since that moment been a wiring standard for many organizations. We will discuss this standard in detail here.

The RS-232 standard 9600bps port will drive 13 metres of shielded cable. RS232 standard is an asynchronous serial communication method. The word serial means, that the information is sent one bit at a time. Asynchronous tells us that the information is not sent in predefined time slots. RS232 sending of a data word can start on each moment. If starting at each moment is possible, this can pose some problems for the receiver to know which is the first bit to receive. To overcome this problem, each data word is started with an attention bit. This attention bit, also known as the start bit, is always identified by the space line level. Directly following the start bit, the data bits are sent. Data bits are sent with a predefined frequency, the baud rate. Both the transmitter and receiver must be programmed to use the same bit frequency. After the first bit is received, the receiver calculates at which moments the other data bits will be received. It will check the line voltage levels at those moments. With RS232, the line voltage level can have two states. The on state is also known as mark, the off state as space. No other line states are possible. When the line is idle, it is kept in the mark state. For error detecting purposes, it is possible to add an extra bit to the data word automatically. The transmitter calculates the value of the bit depending on the information sent. The receiver performs the same calculation and checks if the actual parity bit value corresponds to the calculated value. The stop bit identifying the end of a data frame can have different lengths. Actually, it is not a real bit but a minimum period of time the line must be idle (mark state) at the end of each word. On PC's this period can have three lengths: the time equal to 1, 1.5 or 2 bits. 1.5 bits is only used with data words of 5 bits length and 2 only for longer words. A stop bit length of 1 bit is possible for all data word sizes.

Goals of the Yost device wiring standard

The mess with RS232 wiring is widely known. It was the reason for starting this website. Dave Yost wanted to solve that mess once and for all, reaching as much as possible of the following goals:

1. All cable connectors should have the same connector type (RJ45)
2. All cable connectors should have the same connector gender (male)
3. DTEs and DCEs should have the same connector wiring
4. All cables should be identical (except for length)
5. No need for null modems or other special cables for specific situations

These goals are very close to the goals DEC wanted to achieve. The Yost standard has however one basic advantage. Because RJ45 connectors are used, eight pins are available which makes it possible to transfer almost all RS232 signals. Therefore the Yost standard can be used with much more equipment.

Yost DTE adapter wiring

Now we know how the cables are wired, it is time to define the adapter wiring for various equipment. Depending of the type of equipment, DB9 or DB25 connectors are used. Layouts for both connectors to a RJ45 socket for DTE equipment is shown here. The colors are defined by the Yost standard. The DTR to DSR connection is optional. Please use the manual of the device or software to decide if this loop is necessary. It doesn't harm most of the time if you connect both lines, even with systems that don't use the DSR input signal.

Test COM port by using HyperTerminal.

The HyperTerminal application has been distributed with the Windows operating system versions for a long time now, and for administrators and technical support Representatives, it can be a very useful tool. HyperTerminal allows a user to make a connection to a "host" system from a Windows computer using an available COM port. This will enable you to verify whether or not a port is active and open.  If you have never looked at HyperTerminal, take a couple of minutes to read through the following and see how it can make your life easier.

The HyperTerminal application is started by default from the Start | Programs | Accessories | Communications | HyperTerminal location. When you start HyperTerminal, you are asked to name the connection you are about to configure. This is useful as once you have configured your connection, you can then save all the settings to a configuration file of the same name. This configuration file can be used to implement equivalent settings for subsequent connections. After selecting a connection name, click OK.

On the Connect To dialog box, you are introduced to the different types of connection that HyperTerminal offers. By default, a dial-up connection using a modem is selected (assuming you have a modem present). If you have installed an external modem in addition to an internal modem that modem should also be present in the drop down menu as a choice.

 If you click the downwards arrow on the Connect Using field, you may see one or more COMx (where x is the number of the COM port. i.e COM5) options depending on the number of serial ports available on your computer. The COMx options are typically used for attaching to something like a UNIX computer via serial cable or to a router via its serial console cable. 

To test a specific COM port select that COM port you wish to test. Once the COM port is selected you will not be able to access the other options on this dialog box. They will appear grayed out.

Click OK and select these options:

9600 Bits Per Second, 8 Data Bits, No Parity, 1 Stop Bit, and Hardware Flow Control.

Before clicking OK on the COM3 Properties Dialog Box look at the lower left corner of the HyperTerminal Window. Notice it says "Disconnected" See graphic Below.

Now click the OK button on the COM3 Properties Dialog box. Watch the lower left corner of the HyperTerminal Windows. If the COM port is available and can be opened you will see the status change to Connected. See graphic below.

 If you select OK and get an error saying "Unable to open COMx (where x is the COM port number). Please check your port settings". The COM port you are testing is being used by some device or is not functioning correctly.

Start at the beginning of the COM port test and test another available COM port.

If you receive the error we discussed on every port you select then there are no available ports and you will need to either troubleshoot further or speak to your hardware manufacturer and ask your manufacturer to recommend a hardware solution appropriate for your situations.

Test COM port by using Loopback tester

This is a simple and useful tool for testing RS-232 ports in DTE equipment are working working or not. This plug is connected so that every sent character is echoed back.

 If you Short DB9 (Com Port / RS232) Pin 2 & 3, & Press any Word via Keypad, you can get Eco of that Key. IF you got replied then your Com port is Working Normal, IF not then need to either troubleshoot further or speak to your hardware manufacturer and ask your manufacturer to recommend a hardware solution appropriate for your situations.

Differences between RS-232 and full-duplex RS-485

From a software point of view, full-duplex RS-485 looks very similar to RS-232. With 2 pairs of wires -- a dedicated "transmit" pair and a dedicated "receive" pair (similar to some Ethernet hardware), software can't tell the difference between RS-485 and RS-232.

From a hardware point of view, full-duplex RS-485 has some major advantages over RS-232 -- it can communicate over much longer distances at higher speeds.

Alas, a long 3-conductor cable intended for RS-232 cannot be switched to full-duplex RS-485, which requires 5 conductors.

RS-232 is only defined for point-to-point connections, so you need a separate cable for each sensor connected to a host CPU. RS-485 allows a host CPU to talk to a bunch of sensors all connected to the same cable.

Differences between RS-232 and half-duplex RS-485

But a lot of RS-485 hardware uses only 1 pair of wires (half-duplex). In that case, the major differences are

• Each RS-485 node, including the host CPU, must "turn off the transmitter" when done transmitting a message, to allow other devices their turn using the shared medium
• The RS-485 hardware generally receives on the receiver every byte that was transmitted by every device on the shared medium, including the local transmitter. So software should ignore messages sent by itself.

A long 3-conductor cable intended for RS-232 can often be switched to half-duplex RS-485, allowing communication at higher speeds and at higher external noise levels than the same cable used with RS-232 signaling.

RS-232 is only defined for point-to-point connections, so you need a separate cable for each sensor connected to a host CPU. RS-485 allows a host CPU to talk to a bunch of sensors all connected to the same cable.

Alas, half-duplex RS-485 networks are often more difficult to debug when things go wrong than RS-232 networks, because

• When a "bad message" shows up on the cable, it is more difficult (but not impossible) to figure out which node(s) transmitted that message when you have a shared-medium with a dozen nodes connected to the same single cable, compared to a point-to-point medium with only 2 nodes connected to any particular cable.
• Transmitting data bidirectionally over the same wire(s), rather than unidirectional transmission, requires a turn-around delay. The turn-around delay should be proportional to the baud rate -- too much or too little turn-around delay may cause timing problems that are difficult to debug.

Differences between RS-232 and both kinds of RS-485

RS-485 signal levels are typically 0 to +5 V relative to the signal ground.

RS-232 signal levels are typically -12 V to +12 V relative to the signal ground.

RS-232 uses point-to-point unidirectional signal wires: There are only two devices connected to a RS-232 cable. The TX output of a first device connected to the RX input of a second device, and the TX output of the second device connected to the RX input of the first device. In a RS-232 cable, data always flows in only one direction on any particular wire, from TX to RX.

RS-485 typically uses a linear network with bidirectional signal wires: There are typically many devices along a RS-485 shared cable. The "A" output of each device is connected to the "A" output of every other device. In a RS-485 cable, data typically flows in both directions along any particular wire, sometimes from the "A" of the first device to the "A" of the second device, and at a later time from the "A" of the second device to the "A" of the first device.

RS422, RS485, comparison with RS232

RS232 is well-known due to popularity of today’s PC’s, unlike the RS422 and RS485. These are used in industry for control systems and data transfers (small volumes, less than hundreds of Mb/s).

So, what is the main difference between RS 232 and RS 422 & 485? The RS 232 signals are represented by voltage levels with respect to ground.

There is a wire for each signal, together with the ground signal (reference for voltage levels). This interface is useful for point-to-point communication at slow speeds. For example, port COM1 in a PC can be used for a mouse, port COM2 for a modem, etc. This is an example of point-to-point communication: one port, one device. Due to the way the signals are connected, a common ground is required. This implies limited cable length? About 30 to 60 meters maximum. (Main problems are interference and resistance of the cable.) Shortly, RS 232 was designed for communication of local devices, and supports one transmitter and one receiver.

RS422 & 485 uses a different principle: Each signal uses one twisted pair (TP) line-two wires twisted around themselves. We’re talking ‘Balanced data transmission’, or ‘Differential voltage transmission’. Simply, let’s label one of the TP wires ‘A’ and the other one ‘B’. Then, the signal is inactive when the voltage at A is negative and the voltage at B is positive. Otherwise, the signal is active, A is positive and B is negative. Of course, the difference between the wires A and B matters. For RS 422 & 485 the cable can be up to 1200 meters (4000 feet) long, and commonly available circuits work at 2.5 MB/s transfer rate.

What is the difference between RS 422 and RS 485? Electrical principle is the same both use differential transmitters with alternating voltages 0 and 5V. However, RS is intended for point-to-point communications, like RS 232. RS 422 is intended uses two separate TP wires, data can be transferred in both directions simultaneously. RS 422 is often used to extend a RS 232 line, or in industrial environments.

RS 485 is used for multiply-point communications: more devices may be connected to a single cable- similar to e.g ETHERNET networks, which use coaxial cable. Most RS 485 systems use Master/Slave architecture, where each slave unit has its unique address and responds only to packets addressed to this unit. These packets are generated by Master (e.g PC), which periodically polls all connected salve units.

Quick Comparison Chart: