When solving problems in a communications system, the engineer or technician tries to use some standard format to arrive at a faster solution. Industrial communications systems do not always respond to proven approaches that worked with wired inputs and outputs.
Common problems and solutions: Some of the causes of industrial communication problems include:
• There is no power for the station in the network, resulting in a communications failure
• Cable damage, with a resulting interruption in communications
• Grounding and grounding problems that cause intermittent communications failures
• Electrostatic damage to communication ports
• Software accident at one of the stations that causes communications failures
• High levels of electrostatic / electromagnetic interference in the communications link
• High traffic loads on the link, which results in intermittent communications
• Electrical or transient overvoltage through the communications system that results in hardware damage
The impact on the communications system varies from absolute faults (without possible communications) to intermittent communications depending on the severity of the problem. It is possible that intermittent failure is the worst problem, since it is very difficult to diagnose and correct.
General comments on problem solving: Obviously, there is no method of cutting and drying test. It depends on the environment and the history of the system. However, some rules are useful to solve problems of a communication system effectively.
• Extensive and accurate documentation
• Reference reports
• Network simplification
A specific methodology: when solving problems in your communications system, you should follow the following steps:
• Verify that all network communication stations and devices are turned on and operating
• Verify that all wiring has clean connections.
• Check grounding and grounding configurations.
• Some new devices that work with the same power source may be the cause of the problem
• Check if there has been any change or damage in the shielding of the cables.
• Use the diagnostic packages provided as part of the system to compare the number of packets transmitted to dropped packets.
• Begin eliminating devices that are not critical to the system under investigation.
Since RS-232 is a point-to-point system, the installation is quite simple and all RS-232 devices use DB-9 or DB-25 connectors. These connectors are cheap and allow multiple inserts. None of the RS-232 standards defines which device uses a male or female connector, but traditionally the male connector (pin) is used in the DTE and the female connector (socket) is used in the DCE equipment. This is only traditional and can vary in different equipment. He often wonders why a 25-pin connector is used when only 9 pins are needed. This was done because RS-232 was used before the arrival of computers and, therefore, was used for hardware control (RTS / CTS). Originally it was thought that, in the future, more hardware control lines would be needed, so more pins would be needed. During an RS-232 connection installation, it is important to ask the following questions:
• Is one device a DTE and the other a DCE?
• What is the sex and size of the connectors at each end?
• What is the speed of the communication?
• What is the distance between the equipment?
• Is it a noisy environment?
• Is the software configured correctly (all UART parameters are the same for both sides)?
The RS-485 line controllers / receivers are differential chips. This means that cables A and B are referenced with each other. An ‘one’ is transmitted, for example, when one of the lines is + 5V and the other is 0V. A ‘zero’ is transmitted when the line voltages are reversed. In the systems in operation, the voltages are generally somewhere around +/- 2V with reference to each other. Up to 32 devices can be connected in a system without repeater. Some systems allow the connection of five legs with four repeaters and obtain 160 devices in a system
No matter what extreme care you have taken, there is hardly ever an installation that experiences trouble-free setup and configuration. Some common problems related to Modbus installations are listed below. They can be categorized as either hardware or software problems.
• Hardware problems include mis-wired communication cabling and faulty communication interfaces
• Software (protocol) related issues arise when the controller application tries to access non-existent target devices’ nodes or uses invalid Function Codes, addresses non-existent memory locations in the target devices, or specifies illegal data format types, which obviously the target devices do not understand.
The Modbus Plus network is a 3-wire (one pair and a shield) twisted pair cable with the nodes connected in a daisy-changed configuration. There is no polarity requirement at the node’s transceiver, so the data cable pair may be connected either way at a node. A 220-ohm terminator is required at each end of the network cable. There are limits on the maximum number of nodes per segment, the number of repeaters, and the lengths of cable segments on the Modbus Plus network.
The node address of the Modbus Plus device should be set before connecting it to the network. This avoids possible duplicate address problems with other units on the network. Most software related issues arise from the use of invalid target device addressing; illegal target memory addressing, illegal data formats and even perhaps use of unrecognized function codes. Other issues are related to the actual configuration of the communication hardware itself.
Troubleshooting the wiring of Data Highway Plus: Inspect the cable closely for wiring problems if the operation of the network seems intermittent. Typical problems include:
• Damage to the cable
• Without terminator (150 Ω) at the end of the line
• Screen that is not grounded or damaged.
Data Highway Plus Network Diagnostics: Many of the errors are the result of excessive noise in the network and can be corrected by examining the actual wiring and eliminating the source of noise, if possible. If not (for example, due to
the road (trunk line) parallel to a power cable in a cable tray), the use of fiber cabling as a replacement for the copper cable will have to be considered.
Some errors (identified in the diagnostic registers in the interface module) that are worth mentioning are:
• ACK waiting time
• Fake survey
• Messages transmitted and messages received
• Assignment of data pin.
Keep in mind that the rules for troubleshooting on the physical side of these two cables are very similar to those of RS-485. In fact, DH485 is identical to RS-485, while Data Highway Plus is essentially an isolated version of a transformer. The most difficult part to diagnose problems with Data Highway Plus is in the operation of the protocol.
Beside the actual instruments that require calibration, the only major problem that can occur with HART is the cable length calculation. The HART protocol is designed to work over existing analog signal cables, but it depends on sufficient voltage drop across the series resistor. This, in turn, depends on:
• The load resistance of series
• Cable resistance
• Cable capacitance
• The number and total capacity of field devices
• The resistance and position of other devices in the cycle
The main reason for this is that the network must pass the HART signal frequencies without loss or excessive distortion. A software package such as H-Sim can be used to calculate if the system is operating at the correct signal level. In addition, it must be confirmed that the loop has a bandwidth of at least 2500 Hz. This can be achieved by checking that the product of the resistance and capacitance of the cable (R times C) is less than 65 microseconds.
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Installation & Troubleshooting of communication systems -part 1