Modbus Transceiver Circuit Design Modbus is a protocol, and the physical layer it usually runs on is a shared two wire bus with differential signalling. The protocol stack is a software problem, while the reliability of a Modbus link in a plant depends on the electrical details: biasing, termination, timing…

Cleaning Chemistry Selection for PCB Assembly Lines Cleaning is often treated as a piece of equipment rather than a chemistry, and the choice of cleaning chemistry is made after the machine has been ordered. That order of work puts the process in a corner, because the chemistry determines the temperature,…

Photoelectric Sensor Interface Circuit Design A photoelectric sensor detects an object by the change it makes to a beam of light. The optical part decides what can be seen; the circuit decides whether the change can be separated from sunlight, from a dirty lens and from the drift of its…

Copper Thickness Measurement on Inner Layers of a PCB Inner layer copper carries the current, forms the reference planes and sets the impedance of every controlled line on the board, yet its thickness is one of the least visible properties of a finished panel. It is buried under laminate and…

RS-232 Serial Port Circuit Design RS-232 is one of the oldest interface standards still in daily use, and it survives because it is simple, robust and understood by every platform. It is also an unusual interface electrically: the signals swing between positive and negative voltages, the logic is inverted, and…

Reed Switch Interface Circuit Design A reed switch is two ferromagnetic blades sealed in a glass capsule, closed by a magnetic field. It needs no supply, works from millivolts to hundreds of volts and survives millions of operations, which is why it still appears in door sensors, flow meters, level…

Thermopile Sensor Interface Circuit Design A thermopile sensor measures infrared radiation from a surface and turns it into a small voltage, typically tens of microvolts per degree of temperature difference. Because the signal is tiny and the sensor also responds to its own temperature, the interface has to solve a…

Press Fit Connector Design And Assembly Practice A press fit connector makes its connection by deforming a pin inside a plated hole, so no solder is involved and no heat is applied. The technique suits a backplane that carries many connectors and cannot be passed through a soldering process, and…

Via Tenting And Plugging Design Rules A via is a hole through the board, and unless it is covered it is also an opening that solder, flux and cleaning fluid can enter. The decision about what to do with it is made in the fabrication notes, and it is often…

PCB Drill Aspect Ratio And Hole Quality Limits Every plated hole in a board begins as a drilled hole, and the drilling process imposes limits that the design has to respect. The most important of those limits is the ratio of the hole depth to its diameter, because it governs…