The Tech Blog

I was planning to do some home networking and happened to run across SFPs. Let’s see those in detail today. SFP stands for Small Form-factor Pluggable, and it’s essentially a hot-swappable transceiver module that plugs into a switch or router port to adapt to different media types.

All of would know the classic RJ45 connectors, that can be used for connecting Copper cables(Cat5e) to network interfaces. As networks grew beyond 100m and speeds pushed past 1 Gbps, a better interface became necessary. Around the early 2000s, major vendors agreed on the SFP, so different manufacturers could supply compatible modules. In SFPs, you get a slot where you can choose a copper module for short runs or an optical fiber module for longer distances, even mixing multimode and single-mode fibers. That was a game-changer for cost and flexibility. SFP is a port that eases the life a bit for the designer, as they really don’t need to take care (to an extent) if it’s Copper or Optical cable on the end. No board respin needed.

Physically, the SFP “cage” connector is larger than RJ45. The SFP module fits inside the connector and has the interfacing circuitry required, freeing the host board from that footprint. It’s hot-swappable, meaning if a copper port fails, you replace just the module without powering down the device.

In practice, you will use for a copper SFP module when I need to extend a cable-run just beyond the 100 meter RJ-45 limit. For anything over a few hundred meters, or if you need immunity from electrical interference, fiber SFP (or SFP+) is the answer.

SFPs come in different types considering the speed class they cater to you. Look up on wiki for the full list. There are variations like QSFP (Quad SFP) and QSFP28, catering to 100 Gigabits of transfer speed. These are the things that move data around in data centres. In essence, SFP allows for a host of things as a pluggable interface.

I think the 4–20mA current loop is one of the simplest and most reliable ways to send analog sensor data over long distances. I’ll share a bit of history, why those spans exist, and some practical tips from my experience.

The 4–20mA loop dates back to 1950s process industries. The idea of a current loop really took hold when plants had to move from using pneumatic control (3–15 psi, purely mechanical) to electronic signals. Engineers needed a signal that could travel long over noisy wires without degrading. It was expensive to detect signals below 3psi hence the lower range of 3psi got stuck. By driving a constant current rather than a voltage, you avoid voltage‐drop issues in the field wiring: no matter what resistance the wires add, the receiver always sees the same current. BTW, lower limit is 4mA and not 0mA because of “live zero” measurement. It means if we use 0mA we can’t detect if the value is actually meant to zero or the loop is broken because of power loss or wire-break in circuit.

Technically, a 4–20mA loop is just that: the sensor or transmitter adjusts its internal resistance so that it draws between 4mA (minimum) and 20mA (maximum) from a fixed 24V DC supply. The controller reads that current across a shunt resistor (commonly 250Ω, so 4mA→1V, 20mA→5V) into an ADC(If its Vref is in that range). Why 20mA at the top end? It’s a practical limit. Early transistors and relays tolerated up to around 20mA without burning out or dropping too much voltage.

In practice, you’ll see 4–20mA used for pressure transducers, temperature transmitters, flow meters, level sensors, pH probes, and almost any industrial instrument.  When practically implementing circuits for 4-20mA, make sure to take care of ground loops and to have isolation circuit in the front end of your receiver, purely as you are working in industrial environments. There are a lot of ready-made ICs from major suppliers to help build the 4-20mA loops. Search online and pick from them.

Fun fact: you can layer HART digital communication onto a 4–20mA loop without disrupting the analog measurement, allowing for diagnostics.