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Introduction
Advantech's ADAM serial I/O modules are designed for half-duplex RS-485 serial communication only. Most industrial engineers are familiar with the three more common serial communication standards, RS-232, RS-422, and RS-485. Before the recent popularity of USB, consumer computer systems were equipped with at least one RS-232 port, but rarely RS-422 or RS-485. However, industrial computers are commonly equipped with RS-422 or RS-485 ports. These have long been the preferred standards for industrial applications because they use a more noise immune differential signal allowing much greater cable length between devices than RS-232. Of these, only RS-485 is specified as a half-duplex multi-drop standard.

Serial communication technology is broad and complex. To help assure a degree of compatibility between manufacturers, standards have been established. The current standard for ADAM serial I/O module communication is EIA/TIA-485. The Electronic Industries Association, EIA, and the Telecommunication Industry Association, TIA, have officially replaced the prefix "RS", meaning Recommended Standard, with "EIA/TIA". But for the sake of this simple explanation the more familiar term, RS-485, will be used. Establishing the distinctive benefits of RS-485 over other common standards like RS-232 and RS-422 only requires a basic and generic description of the technology. RS-485 offers two significant characteristics of its technology that make it best suited to the ADAM serial I/O modules. The "differential signal" technology, also called "balanced line" is the characteristic that makes RS-485 preferred over RS-232. The "multi-drop" technology of RS-485 makes it preferred over RS-422 and RS-232.

Differential Signal
Differential signal or balanced line technology is used in both RS-422 and RS-485, unlike RS-232 which uses a single ended signal. Single ended means the data signal references signal ground. For example, Advantech RS-485 based ADAM I/O uses the ASCII character '$' to delimit, or begin many of its command strings. To simply send the ASCII character '$' over an RS-232 link with a common configuration of 8 data bits, no parity, and one stop bit, the electrical signal might look like this:

Figure 1:


Keep in mind that RS-232 is a peer-to-peer communication standard which means only two devices communicate on the link. Therefore, the transmit line coming from one device is connected to the receive line of the other device, and vice-versa on a separate conductor. For this reason, RS-232 is often described as a 3 wire system since two data lines are required along a common signal ground conductor.

The distance an RS-232 signal can be transmitted is dependant on several electrical characteristics. First, the strength of the RS-232 transmitter and the discriminating sensitivity of the receiver are significant. This depends upon the quality of the electronic components. Second, the quality of the cable used for the transmission of the RS-232 signal is highly significant. The EIA/TIA-232-E standard specifies a maximum total system load capacitance of 2500 pF, but not a specific cable length. Third, the baud rate or frequency of the signal is significant and also typically easily adjusted by the user. Fourth, the electromagnetic interference, EMI, in the environment surrounding the devices or the cable can be a very significant factor. Industrial environments can have very high levels of EMI.

All of these factors are significant for both RS-232 and RS-485. But, because RS-485 uses a differential signal technology, it has higher immunity to EMI.

Because RS-232 compares the data signal to the signal ground, stability of the voltage on the ground conductor as well as the data signal line is extremely important. If EMI or a floating ground causes the ground to fluctuate even by as little as a few volts, it can disrupt the signal. Look at figure again. The curved leading edge of what ideally would be a square wave is due to the capacitance of the system, predominantly the wire. This is why the quality of the wire is so important for all serial communication. The tick marks represent the time interval defined by the baud rate.

For this illustration the peak voltages for the data signal are nominally +/- 5V. However, the deadband for transition is typically between +3V and -3V relative to signal ground. This means that the RS-232 receiver must see the polarity change relative to signal ground, and it must swing from above +3V past -3V going negative relative to ground, or vice-versa to see a data change from 0, "Space", to 1, "Mark", and back. Anything which causes the signal ground or data signal voltage at the receiving end to fluctuate enough to prevent the data signal wave from transitioning through this 6v deadband will corrupt the reception of the data. The common term for this is "noise". To reduce its effect baud rate can be reduced and higher quality wire and shielding can be installed.

The best defense against noise is to use a standard that employs differential signal along with high quality cable and shielding. Figure 2 illustrates the differential signal that matches the signal from the single ended signal in Figure 1.

Figure 2:


With differential signal there are two conductors each carrying the same signal data but with inverse polarities. For this reason RS-485 systems are sometimes called a two wire system. Remember, RS-232 had two data lines, transmit and receive, and signal ground. Because RS-485 does not reference signal ground, but only its inverse signal line, the signal ground conductor is not required. However, keeping the voltage of logic ground on the various RS-485 devices relatively close is important. This is typically referred to as the Common Mode voltage range and is nominally -7V to +12V.

The most important aspect of differential signal transmission is that high quality twisted pair cable is used. Serial data transmission is relatively high frequency alternating current. As a result the signal itself will tend to create noise by inducing a current into other conductors in very close proximity, such as in the same cable. This is a type of noise frequently called cross-talk". In RS-485 this is a desirable characteristic and used to an advantage. The two inverted signals are purposely opposed and the conductor pair that carries these signals are twisted throughout the cable so that they induce into each other. The general effect is that the induced signal tends to reinforce the inverse signal in the conductor into which it is induced.

The RS-485 signal is called differential because the receiver compares the positive data line voltage to the negative data line voltage and looks for a difference in potential. Referring to the data lines as negative and positive is a little misleading since both signal lines vary between 0V and nominally +5V. But the signal lines always have opposing polarities relative to each other except when crossing in transition. Instead of a comparison to an assumed stable signal ground as in RS-232, RS-485 only compares the two signal lines. This is very important to noise immunity. If there is EMI present around the cable, it will tend to induce the same current into both signal lines. So if the positive data line voltage increases, the negative data line voltage tends to increase by the same amount and the voltage differential between the two lines essentially remains the same. The differential signal technology adds such stability to this mode of serial communication that with the proper cable RS-485 communication can be reliably extended to 4,000 feet where engineers are reluctant to run RS-232 more than 25 feet in an industrial environment.

A word of warning. Although devices using differential signal are more immune to noise, this is not additional protection against higher voltage on the signal lines such as electrostatic discharge, ESD. High enough voltage on the signal line will always damage communication devices. ADAM serial I/O modules are protected with optical isolation. This prevents high voltage from transferring through the module from the communication line to the real I/O or vice-versa. But, optical isolation does not protect the front end transmitter and receiver circuitry before the optical isolators. Proper precautions should always be taken to prevent exposing serial communication lines to excessive voltage or induced currents.

Multi-drop
Both RS-485 and RS-422 are differential signal standards and are more reliable than RS-232 over longer distances and in noisier environments. But, only RS-485 is designed for use in a serial multi-drop network. By definition RS-422 may be considered multi-drop, but this is only for a simplex network. What this means is that RS-422 transmitters can support up to 10 receivers. Simplex means that the receivers all receive the same data signal at essentially the same time. This may be useful in something like a small network of dumb ASCII annunciators where response from these devices is not expected. So, simplex is basically data going in only one direction.

RS-422 is not well suited for a network where bi-directional multi-drop communication is necessary. RS-485 is designed for this type of serial communication in a half duplex mode. Half-duplex means that the transmit signal and the receive signal travel on the same set of twisted pair conductors. Obviously this cannot occur simultaneously as with RS-232 where a separate transmit and receive line exist for each device. But RS-232 is only peer-to-peer which means there is no chance one device will be receiving data signals from multiple other devices simultaneously. To accomplish full duplex communication over a multi-drop network a sophisticated anti-collision communication protocol must be employed such as TCP/IP over Ethernet. A simpler protocol can be used when only one device will speak at a time. Half duplex communication coupled with a master/slave topology forces this sort of sequential operation.

The master/slave concept is very simple yet vital in RS-485 communication. If any two devices talk simultaneously on the two conductor twisted pair, sometimes called TP, the signal will be corrupted. Remember that both conductors of the TP carry the identical signal, but inverted from each other creating a differential signal. So it is as if the TP were one wire. Putting two signals simultaneously on one wire will cause neither of the signals to be discernible. So there must be rules to define which device on the TP communicates and when. One of the rules defining RS-485 communication is that only the master can initiate communication. Another way of putting it is the master always communicates first. After the master transmits it waits for a predetermined period for a response. Only the slave with the device station address designated in the data signal can respond. If no slave device exists with the corresponding station address then the master receives no response and the master program reacts after a predetermined time-out period.

The slave device in this topology is sometimes called a polled or solicited device. This means that slave devices must be silent until asked to respond. So slave devices never arbitrarily initiate communication. They must be polled specifically and individually by the master. When the master initiates communication it is always directing communication at only one slave device. The slave device at which the master is directing its communication is identified with a numeric station address which is embedded in the data packet being serially transmitted. The RS-485 standard specifies that 32 total devices are supported on a single network. However, this is only an electrical limitation. With amplification of the RS-485 signal using something like the ADAM-4510 RS-485 repeater module, the number of possible modules is limited only by the number of station addresses the protocol supports.

The protocol is the specific format of the data expected by any of the devices on the network. One of the earliest open industrial protocols for RS-485 is MODBUS. It was originally designed to allow multi-drop serial communication between Modicon PLCs and other serial devices supporting the PLC. A few years later Advantech also introduced an open RS-485 protocol called ADAM ASCII to support its ADAM serial I/O modules. Commands from the master device which is typically a host computer typically begin with the ASCII delimiter character, "$", and is followed by the numeric station address of the target slave module, an ADAM serial I/O module. The station address is a two character string of a hexidecimal number between "00" and "FF" which is 0 to 255 decimal. So the master or host can initiate a command string starting with the ASCII characters "$01" which would direct the communication string at the ADAM serial I/O device with the station address of 1.

All of the slave devices are listening or in a receiving mode from the RS-485 network when the master transmits the string beginning with "$01" but only the slave device with the station address 1 will respond. It is important to understand that this is the nature of a protocol. All of the slaves are pre-configured to understand a protocol, in this case ADAM ASCII. If a string is transmitted by the host that does not begin with an appropriate delimiter such as "$", all of the slave devices will ignore this data packet. It is the job of the user to insure that the host transmits data in the appropriate protocol format for that slaves to understand. This is why open protocols are important, so that all of the information regarding the protocol syntax is published and anyone can then create programs on the host to control the RS-485 slave devices.

Every slave on the RS-485 network must have a unique station address. In this way, when the host includes the target station address in the command data packet, only one slave device will respond to the command. The half duplex mode is maintained because only the master will initiate communication. The user must not send more than one command at a time through the master so that two slave modules will not attempt to respond simultaneously. The master than waits after each single command for the slave to respond. By using this ordered set of rules the TP never has more than one device communicating on the network at a time. The normal state of the RS-485 network is with all devices listening or in a receive mode.

The master or host does not have a station number on the RS-485 network. There can only be one master. Once the master initiates communication with a single slave station address the master waits expecting to hear from that station address only. The rest of the slave modules will not respond to the response data packet the individual addressed slave module sends back to the master. It is possible that the master would send a command to an address and not get a response. After a time-out period the master might send a command to a different slave module. If the first slave module had been delayed in responding for an inordinate period of time, but finally did respond, it could transmit simultaneously as the second slave module is also attempting to respond. These types of circumstances can cause communication collisions which will corrupt the data. Prevention of these types of anomalies can only be avoided through careful attention by the user to the creation of a well behaved program for the host.

Since the host does not have a station address on the RS-485 network, but may be a computer equipped only with an RS-232 port, a common RS-232 to RS-485 converter may be required such as the ADAM-4520. When using ADAM serial I/O Advantech recommends using ADAM converters to insure electrical compatibility on the network.

However, the popular ADAM I/O modules are now also available with Ethernet TCP/IP network communication connection using the common open industrial protocol, MODBUS TCP. Yet the ADAM serial I/O modules remain one of Advantech's most popular product lines, in part because the RS-485 serial communication standard makes them extremely flexible, inexpensive, and easy to implement.

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