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How Fast can you Go?

Lidia Baynes (25-05-2024)


Again the amount of loss depends of the values of the series and parallel resistance and the loss will increase with frequency. For example; at a higher frequency the model's series resistance could increase from 0.5 Ω to 1 Ω (per wire) and the parallel resistance could decrease from 14.52k Ω to 7.32k Ω. Slew rate limiting drivers will reduce the rise time and decrease the ringing of an unterminated line. This will decrease the amount of time to wait for the signal to stabilize, increasing the possible bit rate. This will reduce the reflections on a RS-422 network to some extent, but RS-422 also recommends a termination resistor at the receiver's end of the wires (well it sort of recommends, depending on the "data rate" or the "signal rise time at the load end of the cable"). Hardware handshaking can be added to an RS-485 network by adding extra hardware, software, and wires, but this is outside the RS-485 standard. To add a little extra noise margin, use 680 Ω resistors and you have one of the more common biasing networks used on a RS-485 network. The recommended arrangement of the wires is as a connected series of point-to-point (multidropped) nodes, i.e. a line or bus, not a star, ring, or multiply connected network.


Increasing the frequency will increase the inductive reactance, thereby increasing the series resistance in this model. The first is that if the line is long enough, an AC "ohmmeter" connected to the input of the model will see the "resistance" of the wires as 120 Ω. See the instructions on the converter you are using. 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. As a general rule when the voltage measured (with respect to circuit common) is "high" the binary state is considered to be a 1 and when the voltage is low the state is 0. There are exceptions to this rule (such as differential logic and negative logic), but for the purpose of this discussion we'll talk about normal logic. You would think that this would mean that when a 1 is being transmitted "A" would be high and "B" would be low.


For normal logic when the input of a driver is a 1 or high, the non-inverting output will go high. The signal will eventually stabilize (until the next bit transition) so the slower the bit rate the more time the signal will have to stabilize before it is sampled (usually by a UART). Adding a resistor to the end of the cable that matches the value of the cable's impedance will absorb the signal traveling down the line and reduce or prevent reflection. While it may seem quite rational to assume that an RS-485 driver IC pin that is labeled A matches the RS-485 standard's wire "A", this is actually an assumption and is in fact not the case. Since RS-485 is a multi-point specification, however, this is not necessary or desirable in many cases. There is no hardware handshaking in the RS-485 standard and in most cases it is no longer needed. The first is that RS-485 denies any control of the logic function of the generator and receiver. The purpose of handshaking is for the receiver to tell the transmitting device to "shut up, I'm full of data and will lose anything more you send me".


When an RS-485 driver gets its data directly from a UART (with no added inversion), you would expect the "A" and "B" wires to match the voltages in the RS-485 standard for the voltages on the wires, but they will not (unless the driver inverts its input). If an RS-232 to RS-485 converter is used, things start to get even more murky. Because a mark (logic 1) condition is traditionally represented (e.g. in RS-232) with a negative voltage and space (logic 0) represented with a positive one, A may be considered the non-inverting signal and B as inverting. If the voltage on the RS-232 line is positive or high, the signal is a binary 0. Should this signal be inverted back to a low? 12 V relative to the signal ground. SC, G or reference, the common signal reference ground used by the receiver to measure the A and B voltages. The longer the cable length, the more effect the termination resistor has on the signal level. Therefore the longer the cable length, the more important the termination resistor is to improve signal quality. The longer the unterminated wires are the slower the bit rate needs to be. If you run a data rate of less than 30kBit and don't need long cables, the termination resistors are probably not required.



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