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- Fiber-optics Engineering Books
- Practical Fiber Optics (IDC Technology (Paperback)) - Import It All
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The termination resistor value depends on the cable used and is typically ohms. Values less than ohms should not be used since the driver chips are designed to drive a load resistance not less than 54 ohms, being the value of the two termination resistors in parallel plus any other stray resistance in parallel. These resistors are placed between the lines at the two furthest ends, not on the stubs and reduce reflections.
The main working difference is that EIA is used for 2-wire multidrop half-duplex systems and EIA is for 4-wire point-to-point full-duplex systems. One driver is used as a transmitter and the other is dedicated as a receiver. Because the EIA chips have three states, TX, RX and 60 Practical Industrial Data Networks: Design, Installation and Troubleshooting high-impedance, the driver that is used as a transmitter can be set to high-impedance mode when the driver is not transmitting data. The receiver is left on all the time, so data can be received when it comes in.
This method can reduce noise on the line by having a minimum of devices on the line at a time. Even so, most manufacture procedures are similar. The most common type of connector used on most EIA systems is either a one-part or two-part screw connector. The preferred connector is the two-part screw connector with the sliding box under the screw phoenix type. Other connectors use a screw on top of a folding tab. Manufacturers sometimes use the DB-9 connector instead of a screw connector to save money.
Unfortunately, the DB-9 connector has problems when used for multidrop connections. The problem is that the DB-9 connectors are designed so that only one wire can be inserted per pin. EIA multidrop systems require the connection of two wires so that the wire can continue down the line to the next device. This is a simple matter with screw connectors, but it is not so easy with a DB-9 connector. With a screw connector, the two wires are twisted together and inserted in the connector under the screw.
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The screw is then tightened down and the connection is made. With the DB-9 connector, the two wires must be soldered together with a third wire. The third wire is then soldered to the single pin on the connector. Note: When using screw connectors, the wires should NOT be soldered together. Either the wires should be just twisted together or a special crimp ferrule should be used to connect the wires before they are inserted in the screw connector. The wires get reversed e. Loose or bad connections due to improper installation 3. Excessive electrical or electronic noise in the environment 4.
Common mode voltage problems 5.
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Reflection of the signal due to missing or incorrect terminators 6. Shield not grounded, grounded incorrectly or not connected at each drop 7. Starring or tee-ing of devices i. Verifying that the installers are informed of the proper installation procedures can reduce loose connections. If the installers are provided with adjustable torque screwdrivers, then the chances of loose or over-tightened screw connections can be minimized.
There are five ways that noise can be induced into an EIA circuit. This makes EIA very tolerant to noise. The communications will also fail if the voltage level of the noise on either or both lines is outside of the minimum or maximum EIA specification. Noise can be detected by comparing the data communication being transmitted out of one end with the received communication at the other assuming no broken wire.
If the data is corrupt at the received end, then the noise on that wire may be the problem.
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If it is determined that the noise problem is caused by induced noise on the A or B lines it may be best to move the EIA line or the offending noise source away from each other. Excessive noise is often due to the close proximity of power cables. Another possible noise problem could be caused by an incorrectly installed grounding system for the cable shield.
Installation standards should be followed when the EIA pairs are installed close to other wires and cables. Some manufacturers suggest biasing resisters to limit noise on the line while others dissuade the use of bias resistors completely. Having said that, it is usually found that biasing resisters are of minimal value, and that there are much better methods of reducing noise in an EIA system. The ground level can change when a high current device is turned on or off.
This large current draw causes the ground level as referenced to the A and B lines to rise or decrease. This can cause a device to float in and out of service. Often, if the common mode voltage gets high enough, it can cause the module or device to be damaged. This voltage can be measured using a differential measurement device like a handheld digital voltmeter.
The voltage between A and ground and then B to ground is measured.
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If the voltage is outside of specifications then resistors of values between k ohm and k ohm are placed between A and ground and B and ground. It is best to start with the larger value resistor and then verify the common mode voltage. If it is still too high, try a lower resistor value and recheck the voltage.
At idle the voltage on the A line should be close to 0 and the B line should be between 2 and 6 volts. Most EIA driver chips will fail if this happens.
It usually affects the devices near the end of the line. It can be detected by placing a balanced ungrounded oscilloscope across the A and B lines. The signal will show ringing superimposed on the square wave. A termination resistor of typically ohms is placed at each end of the line to reduce reflections.
This is more important at higher speeds and longer distances. A line that is balanced will have a ballpark balance between the capacitance and inductance on it. If this balance is disrupted, the lines then become affected by noise more easily. If another device is to be added in the middle, a two-pair cable should be run out and back from the device.
The typical EIA system would have a topology that would look something like the following: Figure 4. The end of the wires should be stripped only far enough to fit all the way into the connector, with no exposed wire outside the connector. The wire should be twisted tightly before insertion into the screw connector. Often, installers will strip the shield from the wire and connect the shields together at the bottom of the cabinet.
This is incorrect, as there would be from one to two meters of exposed cable from the terminal block at the bottom of the cabinet to the device at the top. This exposed cable will invariably receive noise from other devices in the cabinet. The pair of wires should be brought right up to the device and stripped as mentioned above.
Both the braided and the foil will provide the same level of protection against capacitive noise. The third choice, armored cable, has the distinction of protecting against magnetic induced noise. Armored cable is much more expensive than the first two and EIA overview 65 therefore braided and the foil types of cable are more popular.
For most installers, it is a matter of personal choice when deciding to use either braided or foil shielded wire. With the braided shield, it is possible to pick the A and B wires between the braids of the shield without breaking the shield. If this method is not used, then the shields of the two wires should be soldered or crimped together. A separate wire should be run from the shield at the device down to the ground strip in the bottom of the cabinet, but only one per bus, not per cabinet.
It is incorrect in most cases to connect the shield to ground in each cabinet, especially if there are long distances between cabinets. Unfortunately, there is very little in generic test equipment specifically designed for EIA testing. The most commonly used are the multimeter, oscilloscope and the protocol analyzer. It is important to remember that both of these types of test equipment must have floating differential inputs.
The standard oscilloscope or multimeter each have their specific uses in troubleshooting an EIA system. Continuity verification 2. Idle voltage measurement 3. Common mode voltage measurement Continuity verification The multimeter can be used before startup to check that the lines are not shorted or open. This is done as follows: 4. Verify that the power is off 5. Verify that the cable is disconnected from the equipment 6.
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Verify that the cable is connected for the complete distance 7. Place the multimeter in the continuity check mode 8. Measure the continuity between the A and B lines 9. Verify that it is open Short the A and B at the end of the line Verify that the lines are now shorted Un-short the lines when satisfied that the lines are correct If the lines are internally shorted before they are manually shorted as above, then check to see if an A line is connected to a B line.
In most installations the A line is kept as one color wire and the B is kept as another. This procedure keeps the wires away from accidentally being crossed. The multimeter is also used to measure the idle and common mode voltages between the lines. It is read between A and B lines and is usually somewhere between —1. If a positive voltage is measured, it is possible that the 66 Practical Industrial Data Networks: Design, Installation and Troubleshooting leads on the multimeter need to be reversed. The procedure for measuring the idle voltage is as follows: Verify that the power is on Verify that all stations are connected Verify that the master is not polling Verify and record the idle voltage at each station If the voltage is zero, then disconnect the master from the system and check the output of the master alone.
If there is idle voltage at the master, then plug in each station one at a time until the voltage drops to or near zero.