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10 Tips to Remember About Serial

10/22/2018

1. Serial cables require grounding 

Describing RS-422/485 communications systems as “two-wire” or “four-wire” is misleading and dangerous. To ensure that common mode voltage does not compromise your data or damage your equipment, an extra wire should always be used to connect the signal grounds. This signal ground conductor is often overlooked when ordering cable. 

A “two-wire” system actually requires three conductors and a “four-wire” system actually requires five. It is possible to obtain cable with a twisted pair and a third conductor, or cable with two twisted pairs and a fifth conductor. But, it is easier to get cable with an extra twisted pair and use one of the extra wires as the signal ground. 

2. Termination is rarely necessary 

Termination increases load on the drivers, increases installation complexity, changes biasing requirements and makes system modification more difficult. It does not make sense to use it in situations where it is not needed. 

Termination is used to match impedance of a node to the impedance of the transmission line. When impedance is mismatched the transmitted signal will not be completely absorbed by the load and a portion will be reflected back into the transmission line. When the source, transmission line and load impedance are equal, these reflections will be eliminated. 

Here is a good rule of thumb: If the propagation delay of the data line is significantly less than one bit width, termination is unnecessary. The propagation delay can be calculated by multiplying the cable length by the propagation velocity of the cable. This value, typically 66 to 75 percent of the speed of light (c), is specified by the cable manufacturer. In a system with 2000 feet (609 m) of data line, a round trip would pass through 4000 feet (1.2 km) of cable. Using a propagation velocity of 0.66 × c, one round trip would be completed in approximately 6.2 microseconds (μs). If we assume that the reflections will damp out in three round trips, the signal will stabilize at 18.6 μs after the leading edge of a bit. At 9600 baud one bit is 104 μs wide. Since the reflections are damped out well before the center of the bit, termination is not required. 

There are several methods of terminating data lines. B+B SmartWorx recommends parallel termination, in which a resistor is added in parallel with the receiver’s “A” and “B” lines in order to match the characteristic impedance of the data line as specified by the cable manufacturer. (120 Ohms is a common value.) This value describes the intrinsic impedance of the transmission line and is not a function of the line length. Do not use a terminating resistor of less than 90 Ohms. 

3. Termination resistors should be used carefully 

Termination resistors should be placed only at the extreme ends of the data line, and no more than two terminations should be placed in any system that does not use repeaters. Note that this type of termination adds heavy DC loading to a system and may overload port-powered RS-232 to RS-485 converters. 

Another type of termination, AC coupled termination, adds a small capacitor in series with the termination resistor to eliminate the DC loading effect.

4. Termination changes the biasing requirements 

Nine out of ten RS-485 systems have no need for termination. If you are dead set on doing it anyway, know this: You’ had better have a soldering iron handy. If you terminate, you have to change the biasing. 

When an RS-485 network is idle (no data is being transmitted) all RS-485 drivers are inactive and in the high impedance state (tri-stated). Receivers are enabled, listening for the next data message. During this time the communications line should be in the idle state (mark, logic 1, VAB › -200 mV) so that when a driver does start sending serial data the start bit transition (from logic 1 to logic 0) can be detected. However, since all drivers are tristated, nothing is holding the communication line in the idle state. Without anything driving the network, the state of the line is unknown – essentially floating. If VAB is less than -200 mV, the logic level is indeterminate (and, internally, the output of the receivers will typically be the value of the last bit received). 

In order to maintain the proper idle state, bias resistors must be included in the communication outputs to force the data lines to the idle state voltage. Bias resistors are nothing more than a pullup resistor on the DATA B(+) line (typically to 5 Volts) and a pulldown resistor (to ground) on the DATA A(-) line. 

The value of the required bias resistance is dependent upon the termination resistance in the system and number of nodes connected. The goal is to generate enough DC bias current in the network to maintain a voltage across the data lines (when no drivers are enabled) that will be interpreted as a mark or logic 1 state. The EIA/TIA-485 Standard specifies that a mark is considered anything greater than -200 mV (VAB). 

To ensure reliable operation, B+B SmartWorx suggests that you design for approximately -300 mV. Bias resistors can be placed anywhere in the network, or can be split among multiple nodes. The actual bias resistance is the parallel combination of all bias resistors in a system. 

5. You can add serial ports to computers that lack them 

It is hard to find a new computer with serial ports these days, as the standard peripheral bus for computers is currently USB. That’s okay in home/office applications. But it is something of a nuisance out in the real world where there is still a huge installed base of serial equipment. In fact, RS-422/RS-485 serial standards are still so useful that the number of deployed devices is expected to keep growing for years to come. 

But, while serial ports may not be standard equipment anymore, there is nothing to stop you from adding them. Desktops will normally have a few internal expansion card slots that can be used to give the machine additional functionality. If your computer does nott have built-in serial ports, just plug a serial port card into one of the expansion slots and you are good to go. Newer serial port cards will use the PCIe expansion bus and card slot but, you can still choose from a wide variety of serial port cards for the older PCI expansion bus as well. Depending upon your application, you may want to add cards with multiple serial ports, isolation or some combination of both. 

6. You can convert and isolate in the same device 

Adding serial capability can be trickier when you are using a laptop. They are portable, which is convenient. But, it is getting harder to add internal capabilities. If you are lucky, your machine has a PCMICIA slot or the newer ExpressCard slot. If that is the case, you can buy a serial port card and install it as easily as you would place a serial expansion card into an open slot in your desktop. 

But, newer laptops not only lack serial ports, they are losing their expansion slots as well. Chances are that your only wired option will be USB. To connect to your serial equipment, you will have to use an external USB-to-serial converter. You will need isolation as well, as USB is notorious for its ground loop issues. That would mean using two external devices – a fairly cumbersome arrangement. You might consider acquiring a USB to RS-232 or USB to RS-422/485 serial converter that includes isolation in the same device. 

7. Serial ports can go wireless 

The serial standard may be getting a little long in the tooth, but that does not mean that it cannot keep up with the younger forms of data communication. If you network-enable your serial device with a wireless serial server. it won’t just talk to laptops; it will talk to smart phones and tablets as well. 

8. Serial repeaters can extend range at low cost 

For very long distances you would ordinarily convert serial data to TCP/IP. But, if extreme range is not relevant, an industrial serial repeater can extend full or half-duplex RS-422/485 signals an additional 4000 feet (1.2 km) with no need for conversion, while simultaneously allowing you to add another 32 nodes to your network.

9. Isolation is important 

The larger a network becomes, both in size and complexity, the more likely it becomes that its various elements will reference different grounds. Protect your installation from ground loops with isolators. These can be standalone devices or incorporated into devices like hubs, servers and switches. 

10. Industrial-grade installations are worth the money 

Network-enabled serial devices are deployed in transportation, agriculture, energy, manufacturing and dozens of other industries. They may communicate via Ethernet, wireless and USB. Connections can be made with any mix of copper cable and fiber optics. But, whatever the application may be, every piece of equipment in a network is a potential weak link. Rest assured that you will live to regret it if you try to economize by using items that you acquired at an office store. 

Start with industrial-grade devices straight from the beginning. Whatever additional expense you may incur at the outset, will be dwarfed by your long-term savings on repair costs and downtime.

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