Block of galvanic isolation of RS-232 interface. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Computers

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There is sometimes a rather large potential difference between the cases of devices connected via the RS-232 interface, for example, a computer and a peripheral device. This happens not only when working with high-voltage installations, but also when conventional devices are improperly or unreliably grounded. The equalizing current flowing in such cases along the communication lines distorts the transmitted signals, and it often disables interface microcircuits, including those located on the computer motherboard. Replacing the latter is not cheap. The proposed optical isolation unit, which transmits all the necessary signals without electrical contact of the connected devices, will help to avoid trouble.

In the described block, the electrical isolation of the RS-232 interface signal receiving and transmitting circuits is achieved using high-speed diode optocouplers and signal conditioning amplifiers at the op-amp. The mutually isolated parts of the block are fed from separate network sources. It is considered inexpedient to use transistor optocouplers powered directly from the interface lines. Firstly, the insufficient speed of most of these optocouplers does not allow to achieve a transmission rate of more than 9600 baud. Secondly, the probability of failure of interface microcircuits as a result of the additional load placed on them increases.

The diagram of the optical isolation node for one interface line is shown in fig. 1. The input signal of standard levels for RS-232 through the protective circuit R1VD1VD2 is fed to the op-amp DA1, connected according to the repeater circuit.

The emitting diode of the optocoupler U1 is connected to the output DA1 by the cathode and is protected from reverse voltage by the diode VD3. Resistor R2 limits the current through the diodes. If the voltage at the node input is negative (which corresponds to the transmission of log. 1), current flows through the emitting diode and the photodiode of the optocoupler U1 is in a conducting state under the action of IR radiation. As a result, the voltage at the inverting input of the op-amp DA2 is greater than at the non-inverting one, and at the output of the node it is negative, as well as at the input. With a positive input voltage (log. 0), the emitting diode of the optocoupler U1 is extinguished, the photodiode is closed. Therefore, the voltage at the output of the node is also positive. Due to the feedback through the resistor R7, the switching thresholds of the decoupling node from 1 to 0 and from 0 to 1 are not the same, which improves noise immunity.

The output voltage levels when using the op-amp indicated on the diagram and the supply voltage of ±12 V are ±10,5 V, which fully complies with the requirements of the RS-232 standard. Resistor R8 is a limiting resistor for an LED installed outside the node in question, signaling the transmitted logic level.

The supply voltages to the input and output parts of the decoupling unit (respectively +12 VI, -12 VI and +12 VII, -12 V II) must be supplied from isolated sources in pairs. Their common chains Common. I and Common. II are also isolated from each other. The printed circuit board of the decoupling node and the location of the elements on it are shown in fig. 2.

OA KR544UD2A can be replaced by KR140UD11, KR140UD18 and others, but it is necessary to make sure that the temporal distortion of the transmitted signals does not exceed the allowable for the desired data transfer rate. The replacement for the AOD130A optocoupler should be selected according to the minimum duration of the rise and fall of the output pulse and the insulation voltage required for the problem being solved.

In one of the options for the decoupling node, a diode optocoupler was used, located inside the K293LP1 microcircuit. Its outputs allow you to connect external circuits to the optocoupler, as shown in fig. 3. Conclusions 7 and 8 are left free. To avoid a breakdown between pins 2 and 4, a hole and a contact pad for pin 3 of the K293LP1 chip on the printed circuit board should not be made. The output itself is removed before installation.

To communicate devices via the RS-232 interface, often only two circuits are enough: RXD (data from a peripheral device to a computer) and TXD (data in the opposite direction). The decoupling block diagram for such a case is shown in Fig. 4. The block consists of two interchange nodes A1 and A2 described above, exactly the same, but included in the above circuits in opposite directions. The XS1 socket is connected directly or by a "modem" (without crossover) cable to the computer's COM port plug, and a peripheral device is connected to the XP1 plug in exactly the same way as if it were connected to a computer without insulation.

Please note that the housings of interface cable connectors often turn out to be connected through the shielding braid of the latter to the housings of the computer and peripheral device. For this reason, the connector housings XS1 and XP1 must be carefully insulated from each other and from the decoupling unit housing (if it is made of metal). Keep in mind that touching two connectors at the same time can result in an electric shock.

Jumpers between the contacts of the XS1 socket are needed to "deceive" the computer, simulating the peripheral signals that come in response to its requests. If a real exchange of control signals is still necessary, the jumpers are removed and one more decoupling node is added to the block for each of the interface lines. In the DCD, RI, CTS, DSR lines (input to the computer), these nodes include A1 in the same way. In the RTS and DTR lines (weekends) - similar to A2. Since DCD and RI lines are relatively rarely used in practice, it is usually sufficient to have six junctions.

Four supply voltages for decoupling nodes are obtained from the isolated windings II and III of the transformer T1 using rectifiers on diode bridges VD1 and VD2. Their values ​​are not stabilized and can be in the range of 11,5 ... 13,5 V (in absolute value).

The power transformer T1 must be given special attention. The insulation between its windings must withstand a voltage not less than that for which the optocouplers installed in the decoupling nodes are designed - 1500 V or more. Windings II and III must be shielded from each other and from winding I, otherwise impulse noise can enter the communication line through parasitic capacitance.

The required voltage can withstand the insulation of only those small-sized transformers, the windings of which are placed on different cores of the magnetic circuit or in separate sections of the frame on one core. However, a ready-made transformer of this design with the necessary windings, and even with a screen between them, is unlikely to be purchased. It remains to choose a suitable overall power and rewind its secondary windings. Preference should be given to a transformer with a relatively free window of the magnetic circuit. This will allow you to place windings with reinforced insulation and a screen without the hassle.

The calculation of new secondary windings is not difficult. With a primary voltage of 220 V and a load current of at least 30 mA, each secondary winding should provide 20 V (with a tap from the middle). By measuring the secondary voltage before reworking the transformer and counting the number of turns of the removed winding during disassembly, it is easy to determine the required number of turns of the new one. It will change in proportion to the voltage. The winding wire is taken with a diameter of 0,1 ... 0,15 mm. It will withstand the required load with a margin, and thinner winding is very inconvenient.

A factory-made transformer is almost always filled with varnish, but with some skill it can still be disassembled without damaging the winding and magnetic circuit plates. I do it this way. Using a knife with a thin blade, I separate the top plate from the set, trying not to damage the windings. In order for the blade to fit inside the central core of the magnetic circuit, it must be narrow enough. The more part of the plate that can be separated, the higher the probability of successful disassembly. Then, not much, but firmly, I clamp the magnetic circuit in a vice (through cardboard spacers) and, using a suitable auxiliary plate of hardened steel, knock out the plate that was left not clamped and separated from the set from the frame.

Further disassembly is usually not difficult. Having finished it, I remove the existing secondary winding from the corresponding section of the frame and wind new ones, not forgetting to provide a screen between them - an open coil of copper foil or a layer of winding wire turn to turn. As insulation between the windings or the winding and the screen, I lay several layers of oiled capacitor paper. It can be "obtained" by dismantling a large-capacity paper capacitor, for example, used in ballasts for fluorescent lamps. Having finished rewinding, I return the plates of the magnetic circuit to their place.

Do not be upset if a few plates are left "superfluous". This will not affect the quality of the transformer. If two secondary windings could not be placed on the frame, two identical transformers can be made, each with one well-insulated secondary winding. Their primary windings are connected to the network in parallel.

Having assembled the unit, you should first of all check the insulation between the circuits of the XS1 and XP1 connectors. An ohmmeter connected between any pin or case of the first and any pin or case of the second connector should show infinite resistance. In critical cases, the insulation is checked with a megger that develops the appropriate test voltage. One of its outputs is connected to the contacts securely connected together and the body of the XS1 socket, the second - in the same way to the XP1 plug. It is necessary to check the insulation of the interface circuits from the mains, as well as from the magnetic circuit and the shield of the T1 transformer.

The first inclusion of the assembled block is carried out without connecting it to a computer and a peripheral device. The voltage is measured on pins 1, 2, 6, 8, 9 of the XS1 socket and on pins 3, 4, 7 of the XP1 plug relative to pin 5 of the corresponding connector. It should exceed +10 V, and when applied to the contact with the same number of the opposite connector, the voltage below -5 V (relative to pin 5 of this connector) will change to negative -10 V or less. At the same time, the corresponding LED should light up. Naturally, only those circuits that are equipped with decoupling nodes in the assembled structure are subject to verification. For example, in the block according to the scheme shown in Fig. 4, just check the voltage between pins 2 and 5 of the XS1 socket and between pins 3 and 5 of the XP1 plug

After making sure that the unit is working, connect it between the computer and the peripheral device and, turning on the power (the first - the computer), using a test or working program, make sure that the data is transferred correctly.

The described block in the six-channel version has been successfully operating for more than a year and a half, providing communication between a computer and a TDS-340 oscilloscope, which is under a potential of 2000 V. The block was also tested when connecting a computer to an industrial controller based on an 18031 microprocessor installed in another room.

The maximum information transfer rate is 19200 baud. There was no need to work at a higher speed, although theoretically such a possibility exists.

Author: N. Maramygin, Moscow

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