Author: fleta

A decent oscilloscope that is isolated from earth ground (battery operated or powered by an unplugged UPS (if you’re good enough with a scope you can also subtract channel 1 from channel 2 to see the differential waveform, or use a differential probe, etc.)) can be a very useful troubleshooting tool. It also defines three generator interface points (signal lines); A, B and C. The data is transmitted on A and B. C is a ground reference. RS-485 simply defines the interface connection points as “A” and “B” and shows the voltage relationship between “A” and “B” for the binary states of the two wires, not the binary state of the input to the driver. This is not technically correct since RS-232 requires the use of voltages outside those of normal logic, but the input and output of an RS-232 driver look inverted on an oscilloscope, and most data sheets show the driver as an inverter. Biasing has a number of uses on a RS-485 network, but first lets look at what RS422 and RS-485 have to say.

This is a good first step if the communications are not working. RS-485, also known as TIA-485(-A) or EIA-485, is a standard, originally introduced in 1983, defining the electrical characteristics of drivers and receivers for use in serial communications systems. It may be used to control video surveillance systems or to interconnect security control panels and devices such as access control card readers. These characteristics make RS-485 useful in industrial control systems and similar applications. Many other IC manufacturers make a drop-in replacement for this IC, and many of them (such as the MAX483) use the same labeling. You can design the system to accomplish this if you pay careful attention to logic function of the driver being used, as well as the labeling of the terminal on the device. This provides some noise immunity as well as resolves the problem of missing the start bit, but only for the receivers that implement this internal biasing.

Lowering the bias resistor values will increase the bias voltage which will increase the noise immunity at the cost of increased current from the 5V supply. Normally you will want to use a pull-up voltage that matches the driver’s supply voltage. When the input is low or 0, the non-inverting output will go low. The inverting output usually is shown with a bubble on it and the non-inverting output does not have the bubble. When the input is high the inverting output goes low, and when the input is low the inverting output goes high. Conversely, the output that goes low when the input goes high is often called the “inverting” output. This is why long cables and high frequencies are incompatible. RS-422 is similar (except they leave the hyphen out) Both standards have a section defining why you may want fail-safe operation, but neither discuss how to implement it.

This is why RS-485 states “The use of a cable termination is normally required”.. Unfortunately RS-485 section 4.5.3 states that the total load between “A” and “B” should be no less than 54 Ω. Therefore the total bias resistance is effectively in parallel with the termination resistor as a load to the cable. So it does not matter where the driver is connected on the cable, it “sees” a 60 Ω load. Should also be used on RS-485 networks if the driver is always enabled. By using repeaters very large RS-485 networks can be formed. See the instructions on the converter you are using. There are a number of other things to note with this model. It should be noted that this model is only valid for one frequency. One of the pairs is dedicated to PC-to-peripheral communication. A rule of thumb is that if the propagation delay of the data line is much less than one bit width, termination is not needed. The longer the unterminated wires are the slower the bit rate needs to be. But the baud rate and bit framing must also match. When a binary 1 (aka OFF or marking) is being transmitted the voltage on TXData must be less than -3V.

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