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RS-485 compliant drivers deliver a minimum differential output of 1.5 V over a 60-Ω load, while compliant receivers can detect a minimum differential input of 200 mV. We connect the generator to the input of the microcircuit, and look at the output signal. The easiest way to protect the microcircuit from short-circuits is to add limiting resistors for each transistor in the output buffer. The ultimate right way around the loopy grounds problem is ground isolation - either at the PSU, or within the device, perhaps preferably at the RS485 port. In my example, I just used another pair in the CAT5 cable for a reference ground. The standard does not discuss cable shielding but makes some recommendations on preferred methods of interconnecting the signal reference common and equipment case grounds. The worst situation, that is, the maximum short-circuit currents can be in the case when the transmitter broadcasts a «logical unit», and the potential of -7 V is consistent in the line, and the second situation is when the transmitter broadcasts a logical 0, and the potential of 12 V is consistent in the line. Most often, this situation occurs when two or more transmitters are "broadcasting" simultaneously. Therefore the longer the cable length, the more important the termination resistor is to improve signal quality.

Termination resistors also reduce electrical noise sensitivity due to the lower impedance. A single set of bias resistors could be used at one end of the network, if the resistance was halved to 720 Ω. In the RS485 interface, when organizing a network, a microcircuit that transmits information is called Master, and the one that receives information - Slave. When wiring a RS-485 network, always connect "A" to "A", "B" to "B", and "G" to "G". The ground between RS-485 devices is often called a "Third Wire". Another approach to managing idle bus conditions involves using two external resistors: one connected from the A terminal to VCC and the other from the B terminal to the ground. In one revision we did just like that, but it turned out to be a fail… The parameters turned out to be comparable with the foreign analogs, which is good news for the developers of the microcircuit. However, achieving good performance is only the tip of the iceberg. This is a good first step if the communications are not working. Half-Duplex RS-232 is usually limited by an additional communications device between the two farthest ends that are communicating. Transistors P1 and N2 will be open, and without additional protection, a short-circuit current will flow through them, which will be limited only by insignificant resistances of the transistors themselves and the conductor.

In K5559IN86ASI, the thermo-sensor response threshold is located around 165 ºС - when this value is reached, the microcircuit will turn off. At elevated temperatures, the parameters of the transistors begin to degrade, and with a short-circuit current, heating is inevitable and, therefore, it is important to turn the microcircuit off in time. The operating temperature range, stated in the documentation, is given relative to the ambient temperature, and, fairly speaking, it is easier to control. The guaranteed temperature range, in which the microcircuit should function: -40… The chamber allows you to set the temperature from -60 ° С to 150 ° С. Here, we use the same heat and cold chamber. RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422. RS-422 specifies the electrical levels of one transmitter to one of more receivers on a single pair of wires. I connected the two conductors in parallel to avoid possible RF effects in the pair. Basic termination networks, such as standard parallel termination networks, typically employ a single resistor. In this case, of course, the characteristics of the microcircuit must fully comply with the requirements of the standard. Such as, in my case, both Zcomm and Zdiff were 100 ohms.

In the simplest case, it consists of 2 diodes used in reverse connection (one to ground, the other to power). The circuit of current mirror determines the maximum current that can pass through the N2 and P2 transistors, thereby limiting both - the forward short-circuit current and the reverse. The reverse current raised the LDO voltage and the circuit burned out. The common-mode loop square is clearly smaller now, so the resonant frequency is higher and the oscillations fade out faster. I connected the ground at both nodes of interest straight to the relevant earth (I didn't install a "separation resistor") - so the grounds actually formed a hard ground loop (closed across the power supplies and mains earth), which traditionally is another big thing to avoid. SC, G or reference, the common signal reference ground used by the receiver to measure the A and B voltages. Clearly owing to the perfect isolation of the meter's RS485 port, combined with the lack of a reference ground terminal. For an off, mark or logic 1 state, the driver's A terminal is negative relative to the B terminal. 5V is shown in the figure since many drivers are powered by 5Vdc. Other pull-up voltages can be used and it is not an absolute requirement that the pull-up voltage match the driver's supply voltage.

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