USB, RS232, RS485 or Wiegand? How to Choose an RFID Reader Interface

The interface is the part of an RFID reader that your software or controller actually sees. It decides how the card number arrives, how far the cable can run, how many readers can share one line and how much code your team has to write. The card technology tells you which reader family to look at. The interface tells you which model in that family will work in your system.

This guide compares the interfaces you will find on RFID readers: USB keyboard emulation, USB virtual COM, RS232, RS485, Wiegand and TCP/IP, plus Bluetooth for mobile use. Distances and limits below are typical values from the electrical standards and common practice. The datasheets of the reader and the host always take priority.

Start with the host, not the reader

First ask what will receive the card number. The answer usually settles the interface.

The interfaces side by side

USB: keyboard emulation or virtual COM

Most USB RFID readers sold for desk use are keyboard-emulation readers, also called keyboard-wedge or USB HID readers. They enumerate as a standard USB Human Interface Device, the same device class as a keyboard, so the operating system’s built-in driver handles them. When a card is presented, the reader types its number, usually followed by Enter, into whatever field has focus.

That makes keyboard emulation the fastest route into existing software: web forms, spreadsheets, POS screens and time and attendance systems accept the number with no changes. The limits are just as clear:

• Input goes to the focused window. Unattended kiosks and background services are better served by a serial port.
• It is one-way. The host cannot tell the reader to beep, flash red or skip a card.
• The host keyboard layout applies. On an AZERTY layout, the top-row keys produce symbols and accented letters unless Shift is held, so a 10-digit number arrives as &é"'( and similar characters. Set the input language to English (US) for the reader’s session.

A virtual COM reader appears as a serial port instead: COM3 on Windows, or /dev/ttyUSB0 or /dev/ttyACM0 on Linux. Your application opens the port and reads the data whether or not it has focus. Some USB-serial chips need a driver; others use the operating system’s built-in class driver.

In our catalogue, most 125 kHz and 13.56 MHz USB readers, such as the L110-U 125kHz USB EM4100 reader and H110-U 13.56MHz USB MIFARE reader, are keyboard-emulation readers. The USB UHF desktop reader/writers, such as the U120-U, use keyboard output by default, with a virtual serial port in a customized version. The Q430-M QR code + RFID access reader offers both keyboard and virtual COM modes on its mini-USB port. The full list is on the keyboard-emulation readers page.

A third USB mode, PC/SC (CCID), lets software send commands to smart cards. It is standard for many NFC and smart-card applications. No reader in our current catalogue is specified for PC/SC. Our keyboard-wedge guide compares the three USB modes in detail.

USB 2.0 limits a cable to 5 m. Beyond that, use a powered hub or active extension, or move to RS485 or Ethernet. Long passive extension cables are a common cause of dropped reads.

RS232: simple point-to-point serial

RS232 (TIA-232) sends data on a TX line and receives on an RX line, both referenced to a common ground, at levels between ±3 V and ±15 V. It connects one reader to one host, usually over a DB9 connector. That makes it the traditional choice for kiosks, industrial PCs, terminals and older software that reads a COM port. Cable length is typically about 15 m; low baud rates and low-capacitance cable can go further.

Points that matter in practice:

• Match the settings. Our RS232 desktop readers default to 9600 bps, with 19200 and 115200 bps selectable. The 13.56 MHz models document 8 data bits, no parity, 1 stop bit (8-N-1).
• Know the frame. The H110-R and H150-R document the frame STX(0x02) Len CardType SNR[5] BCC ETX(0x03). The 125 kHz L110-R, L120-R and L150-R use a frame that starts with STX (0x02) and ends with ETX (0x03). Capture a few reads in a serial terminal, in hex view, before you write the parser.
• RS232 is not TTL. Do not wire a DB9 RS232 reader straight to a 3.3 V or 5 V microcontroller UART. Put an RS232 transceiver in between, or use a reader with TTL output, such as the Q420-M slim QR code + RFID reader and the Q430-M.
• Check TX and RX. On a PC’s DB9 port, pin 2 is RX, pin 3 is TX and pin 5 is ground, and the reader’s TX must reach the host’s RX. A reader lead made for a PC port is already wired this way; a null-modem adapter or a custom cable in between can swap the lines.
• Plan power. A DB9 port does not normally supply power. Our RS232 desktop readers run on 5 V DC. The L120-R, L150-R and H150-R have a USB lead alongside the DB9 cable, so confirm the power arrangement for your host when you request a quotation.

If the host PC has no COM port, a USB-to-RS232 adapter creates one, and the reader works as if it were plugged into a native port.

RS485: long cable runs and shared buses

RS485 (TIA-485) sends data as the voltage difference between two wires, A and B. Noise picked up along the cable affects both wires equally, so the receiver rejects it. A twisted pair can run up to about 1,200 m at data rates up to 100 kbit/s. Two-wire RS485 is half-duplex: devices take turns to transmit.

• Terminate long buses. Fit a 120 Ω resistor at each end of the cable, matched to its impedance. Short runs at 9600 bps often work without termination.
• Daisy-chain, do not star. Run the bus from device to device. Long branches cause reflections and intermittent errors.
• Carry a reference. Receivers tolerate a common-mode range of −7 V to +12 V. On long runs, add a ground or reference conductor alongside the pair.
• A and B labels are not consistent between vendors. If no data arrives, swapping A and B does no harm.
• The standard covers only the wiring. Several readers on one pair need a protocol with device addresses and polling. Confirm that the reader supports addressing before you plan a shared bus.

RS485 is common on turnstiles, elevator controllers, UHF vehicle gates and livestock feeding stations. In our catalogue it is offered on the U610-M to U650-M UHF integrated readers (with RS232, USB and Wiegand), the A660-M 134.2 kHz animal tag panel reader (with RS232), and the Q420-M and Q430-M QR code and card readers for turnstiles and doors. All are listed on the RS232 and RS485 readers page.

Wiegand: the access-control reader interface

Wiegand is the de facto link between a card reader and an access controller, and it is the output on our Wiegand 26/34 access control readers. The reader sends each bit as a short low pulse on one of two data lines: a pulse on D0 is a 0, and a pulse on D1 is a 1. Pulse width and spacing vary by vendor; pulses of about 50 µs, 1 to 2 ms apart, are common. The controller counts the bits and decodes the number.

The 26-bit format carries 1 even-parity bit, an 8-bit facility code (0–255), a 16-bit card number (0–65,535) and 1 odd-parity bit. With only 24 data bits, a Wiegand 26 reader sends part of a card’s ID rather than all of it. An EM4100 card with the ID 3A 00 9C 5E 41 is typically sent as facility code 156 and card number 24129. Wiegand 34 carries 32 data bits and 2 parity bits. See Wiegand 26 vs 34 for the bit layouts.

Know the limits before you choose it:

• One-way only. The controller cannot configure the reader. LED and buzzer control use separate wires.
• No encryption and no supervision. A cut cable looks the same as no card presented. OSDP, a Security Industry Association standard that runs over RS485, adds two-way supervision and an encrypted secure channel. Our current catalogue has no OSDP reader.
• The format must match the controller. The L410-W resin-potted 125kHz Wiegand reader sends Wiegand 26 by default and Wiegand 34 with its purple wire tied to GND. The UHF integrated readers offer Wiegand 26 and 34, and the U610-M also lists Wiegand 98. The Q420-M and Q430-M offer Wiegand alongside serial outputs.
• Distance depends on the reader and cable. Around 150 m with 22 AWG shielded cable is a common industry figure. The L410-W is specified for up to 100 m over shielded cable. On any run, connect the reader’s GND to the controller’s GND so the data lines share a reference.
• Wire colors vary between vendors. Always check the datasheet. Our Wiegand wiring guide covers colors, grounding and common faults.

TCP/IP and wireless readers

TCP/IP readers connect to the site LAN. They send reads to a server, or accept commands over a socket. One server can manage many readers without long serial runs, but you take on network setup: fixed IP addresses, firewall rules and, where possible, a separate VLAN for security devices. In our catalogue, the E-type variants of the UHF integrated readers, U610-N to U650-N, add TCP/IP Ethernet to the Wiegand, RS232/RS485 and USB outputs (on the U620-N, RS485 is confirmed at order).

Bluetooth readers pair with phones, tablets and PCs and usually behave as a keyboard, so the no-code rule of USB keyboard emulation still applies. 2.4 GHz models use a paired USB dongle instead. The H510-B pocket Bluetooth NFC reader (13.56 MHz, with a 125 kHz L510-B version), the U510-B and the U550-M UHF handheld support both. See handheld and Bluetooth RFID readers.

How to choose: an eight-point checklist

1. Receiving device. PC application, browser, Android app, access controller, PLC or server?
2. Focus. Must the software capture reads with no text field selected? Then use virtual COM, RS232, RS485 or TCP/IP, not keyboard emulation.
3. Cable length. Up to 5 m: USB. Up to about 15 m: RS232. Longer: RS485, Wiegand or Ethernet.
4. Number of readers. Several readers on one cable need RS485 with addressing, or TCP/IP.
5. Commands to the reader. If the host must control the LED and buzzer or write tags, pick a two-way interface and a reader whose protocol accepts commands. Our 125 kHz and 13.56 MHz readers are read-only; tag writing is available only on our UHF reader/writers and integrated readers.
6. Number format. Which Wiegand format, or which string (10-digit decimal, hex, with or without Enter), does the host expect? Check it with the RFID card number and Wiegand converter.
7. Electrical noise. Near motors, drives or mains cables, prefer RS485 or Ethernet over shielded cable.
8. Power at the reader. USB bus power is 5 V. The L410-W needs 9–16 V DC, and the UHF integrated panels run from a 12 V DC supply.

Mistakes that look like a broken reader

If the reader still stays silent, work through our troubleshooting checklist.

Unsure which interface your system needs? Tell us your host and cable run: the device or controller that receives the number, the cable length and route, and the card or tag type. We will recommend the interface and a reader model, and the sample can ship configured with that output.