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ACR122U — USB NFC Reader Complete Guide

Quick Summary: The ACR122U is ACS's most iconic and widely adopted PC/SC contactless reader. Bridging ISO 14443 Type A/B, MIFARE, FeliCa, and ISO 18092 NFC tags into standard USB interfaces, it boasts unmatched open-source library support (libnfc, pyscard, mfoc) and serves as the definitive starting point for RFID/NFC academic projects and research.

Often referred to as the "Arduino of NFC readers," the ACR122U is cost-effective, widely available, PC/SC-compliant, and the only model in its class featuring a direct native libnfc driver (acr122_usb).

Unboxing & Hardware Overview​

  • Dimensions & Weight: 98.0 × 65.0 × 12.8 mm, pearl-white ABS casing, 70 g.
  • Antenna: Integrated 50 × 40 mm PCB antenna located beneath the top surface (operating range up to 50 mm).
  • LED: Bi-color LED (Red/Green), software controllable.
  • Buzzer: Monotone buzzer, software controllable.
  • Cable: Integrated USB 2.0 Type-A cable, 1.0 m length.

When a card is successfully detected, the LED flashes green and the buzzer emits a confirmation beep.

Specifications Overview​

ItemSpecification
Core ChipsetNXP PN532 (13.56 MHz)
Supported Standards / CardsISO/IEC 18092 NFC, ISO 14443 Type A & B, MIFARE Classic®, MIFARE Ultralight®, FeliCa
Operating Frequency13.56 MHz
Read/Write Speed106 / 212 / 424 kbps
Host InterfaceUSB 2.0 Full Speed (12 Mbps), CCID compliant
Operating DistanceUp to 50 mm (dependent on tag geometry)
Anti-collisionBuilt-in (one card accessed at a time)
Power SupplyUSB bus-powered, 5V DC (typical 100 mA, max 200 mA)
Dimensions / Weight98.0 × 65.0 × 12.8 mm / 70 g
API SupportPC/SC, CT-API (via PC/SC wrapper)
CertificationsISO 14443, PC/SC, CCID, CE, FCC, KC, VCCI, RoHS, Microsoft WHQL
USB Vendor/Product ID072F:2200
OS SupportWindows, Linux, macOS, Solaris, Android 3.1+
Official DocumentationACR122U Product Page, API Manual (PDF)

Supported & Unsupported Capabilities​

FeatureStatusNotes
Read/Write MIFARE Classic & Ultralight✅ SupportedCompatible with libfreefare and nfc-mfclassic
Read/Write ISO 14443-4 Cards✅ SupportedStandard PC/SC APDU transmission
Read/Write FeliCa & ISO 18092 Tags✅ SupportedHandled via pseudo-APDU commands
Read Card Serial Number (UID)✅ SupportedStandardized FF CA 00 00 00 APDU call
Card Emulation Mode❌ Not SupportedDedicated reader/writer firmware
ISO 15693 (Vicinity Cards)❌ Not SupportedRequires ACR1552U
SAM (Secure Access Module) Slot❌ Not SupportedRequires ACR1252U or ACR1552U

Installation & Driver Setup​

The driver architecture operates across two potential paths:

Kernel Module Conflict on Linux

The Linux kernel's default pn533 and pn533_usb drivers automatically claim PN532-based devices, blocking pcscd and libnfc. Blacklisting these kernel modules is required for reliable operation:

sudo modprobe -r pn533_usb pn533
echo "blacklist pn533_usb" | sudo tee -a /etc/modprobe.d/blacklist-nfc.conf
echo "blacklist pn533" | sudo tee -a /etc/modprobe.d/blacklist-nfc.conf

Step 1: Install PC/SC Tools (Debian / Ubuntu / Kali)

sudo apt update
sudo apt install -y pcscd pcsc-tools libpcsclite-dev

Step 2: Start and Enable pcscd Service

sudo systemctl enable --now pcscd

Step 3: Verify Reader Detection

pcsc_scan

Expected output:

Scanning present readers...
0: Advanced Card Systems Ltd. ACR122U 00 00
Waiting for the first reader...

When a contactless card is presented:

Card state: Card inserted,
ATR: 3B 8F 80 01 80 4F 0C A0 00 00 03 06 03 00 01 00 00 00 00 6A

Method B: Direct libnfc Stack​

Step 1: Install libnfc and Utilities

sudo apt install -y libnfc-bin libnfc-dev libfreefare-bin

Step 2: Configure libnfc (/etc/nfc/libnfc.conf)

device.name = "ACR122U"
device.connstring = "acr122_usb:072f:2200"

Step 3: Run nfc-list

nfc-list

Expected output:

nfc-list uses libnfc 1.8.0
NFC device: ACS / ACR122U PICC Interface opened
1 ISO14443A passive target(s) found:
ISO/IEC 14443A (106 kbps) target:
    ATQA (SENS_RES): 00  04
       UID (NFCID1): a1  b2  c3  d4
      SAK (SEL_RES): 08

Quickstart: Reading Card UID with Python (pyscard)​

pip install pyscard
from smartcard.System import readers
from smartcard.util import toHexString

# 1. Connect to reader
r = readers()
if not r:
    print("No PC/SC readers detected.")
    exit(1)

reader = r[0]
print(f"Connected to: {reader}")

connection = reader.createConnection()
connection.connect()

# 2. Transmit standard Get UID APDU: FF CA 00 00 00
GET_UID = [0xFF, 0xCA, 0x00, 0x00, 0x00]
data, sw1, sw2 = connection.transmit(GET_UID)

if sw1 == 0x90 and sw2 == 0x00:
    print(f"Card UID: {toHexString(data)}")
else:
    print(f"Command failed with status: {sw1:02X} {sw2:02X}")

Operating System Compatibility​

OS / PlatformCompatibilityDriver / Notes
Windows 10 / 11✅ SupportedPlug & Play via Microsoft CCID driver
Ubuntu / Debian / Kali✅ Supportedpcscd + libpcsclite (requires kernel module blacklist)
macOS✅ SupportedBuilt-in Apple CCID / PC/SC framework
Android✅ SupportedVia USB OTG and Android CCID libraries
Raspberry Pi / Jetson✅ SupportedStandard ARM Linux packages

Troubleshooting​

1. pcsc_scan shows no readers​

  • Cause: Kernel module collision (pn533_usb claiming device) or pcscd daemon inactive.
  • Fix: Verify blacklist in /etc/modprobe.d/blacklist-nfc.conf and restart sudo systemctl restart pcscd.

2. nfc-list returns "Unable to open NFC device"​

  • Cause: pcscd and libnfc competing for exclusive USB claim.
  • Fix: Stop the PC/SC daemon temporarily when executing raw libnfc tools: sudo systemctl stop pcscd.

3. Frequent card read disconnects​

  • Cause: Insufficient USB port power delivery or USB hub dropouts.
  • Fix: Connect reader directly to rear motherboard USB ports.