ACR122U NFC Reader

Supply: Proud Tek supplies the ACR122U; confirm the quantity when you order. ACS lists it as discontinued, so a new design that has to stay in production for years can start on one of ACS's current USB NFC readers instead, the ACR1252U (USB NFC Reader III) or the ACR1552U (USB NFC Reader IV), which we supply as well; the comparison further down lists what to check in your PC/SC code before switching. If your software is not written yet and you would rather call library functions than send APDUs, the µFR Classic CS with its free SDK is the other route.

What the ACR122U is

The ACR122U is a USB desktop reader/writer made by Advanced Card Systems (ACS). ACS describes it as PC/SC and CCID compliant, for ISO 14443-4 Type A and B cards, MIFARE, ISO 18092 (NFC) tags and FeliCa at 13.56 MHz. Inside, an NXP PN532 contactless controller sits behind a second controller that presents the USB CCID interface, so the host sees a standard contactless smart-card reader named "ACR122U PICC Interface". Its red/green LED and buzzer can be driven from your application.

Three things follow from that design. Software written for PC/SC, such as a pyscard script or a WinSCard application, runs without a vendor SDK. Cards that do not take ISO 7816-4 APDUs, such as MIFARE Classic, are reached through commands with class byte FF, which the reader translates into card commands (see the command table below). And the reader works with one card at a time: its anti-collision selects a single card, so present cards one by one.

Host setup on Windows, macOS and Linux

The Linux pn533 conflict. The Linux kernel has its own NFC driver, pn533_usb, and its device table includes the ACR122U's USB ID (072f:2200). The kernel binds the reader as soon as it is plugged in, before pcscd can: the CCID driver in pcsc-lite then fails with "Can't claim interface", and libnfc cannot open the device either. Unload the pn533_usb and pn533 modules, blacklist nfc, pn533 and pn533_usb in /etc/modprobe.d (libnfc ships exactly that blacklist file), then replug the reader and restart pcscd. Blacklisting disables the kernel NFC stack, so do it only on hosts that do not use it for another device.

PC/SC and ACS commands your software sends

ISO 14443-4 cards such as DESFire and Java Cards take their own ISO 7816-4 APDUs, which the ACR122U passes through. Other cards are reached through commands with class byte FF, and FeliCa and Topaz tags also accept their native commands. FF CA, FF 82, FF 86, FF B0 and FF D6 are the storage-card commands defined in Part 3 of the PC/SC specification, so MIFARE Classic code built on them can move to another reader that implements them, although key slots and supported cards vary by reader. The value-block commands FF D7 and FF B1 are ACS's own, and so are the FF 00 reader commands, which ACS calls pseudo-APDUs. All are documented in ACS's ACR122U Application Programming Interface (V2.04). The ones most integrations need, in hex:

With a card on the reader, send these as ordinary transmits (SCardTransmit) to the ACR122U PICC Interface. Reader commands such as the LED, firmware version or polling settings can also be sent before a card is presented, but only through the PC/SC escape channel (SCardControl with SCARD_CTL_CODE(3500) on Windows), which stays off until you enable it as shown in the host-setup table.

Cards it handles, and cards it cannot

Libraries and tools that work with it

• pyscard (Python), the WinSCard API (C, C++ and .NET on Windows) and javax.smartcardio (Java) are PC/SC layers: open the ACR122U PICC Interface, send the APDUs above as transmits, and send reader commands through the control call once the escape channel is enabled.
• libnfc has two drivers for the reader: acr122_pcsc, which goes through pcsc-lite, and acr122_usb, which opens the USB device directly. Either way the kernel's pn533 modules must not be loaded. For NFCIP or card-emulation modes with no card on the reader, libnfc's README says to allow the CCID Exchange command in libccid (ifdDriverOptions 0x0001). The acr122_usb driver claims the reader's USB interface itself, as the CCID driver in pcscd does, so only one of them can hold the reader: stop pcscd while you use acr122_usb, or use acr122_pcsc.
• nfcpy lists the ACR122U in its hardware table, with read and write support for NFC Forum Type 2, 3 and 4 tags and as a peer-to-peer initiator, but without tag emulation or the peer-to-peer target role. Its documentation advises against buying the ACR122U for nfcpy, because the CCID controller in front of the PN532 prevents full use of the chip; for nfcpy work, choose a reader that nfcpy supports without these limits.

Common pitfalls

• Keys disappear after unplugging. Keys loaded with FF 82 live in volatile memory, so load them at the start of every session. Authenticate again whenever you move to a block in another sector.
• Writing a sector trailer. The last block of each MIFARE Classic sector holds key A, the access bits and key B, so a write to that block replaces the keys and access bits. Try trailer writes on a spare card first.
• A beep on every tap. The buzzer sounds on each card detection by default; FF 00 52 00 00 turns it off.
• Long or large commands. The CCID driver's reader list notes that the ACR122U supports no extended APDUs and no time-extension requests, and records faults in some firmware versions. Test your largest command and slowest card operation, and note the firmware version with the result.
• MIFARE Classic 4K and NDEF. ACS advises a 2-second delay when reading NDEF messages from MIFARE Classic 4K cards.

How to test a sample

1. Connect the reader to the computer and operating-system build you will deploy, and check that the host sees it: Device Manager on Windows, lsusb and pcsc_scan on Linux.
2. Read the firmware version with FF 00 48 00 00, and record it with the operating system and driver version.
3. Place one sample card on the reader and read the ATR. For memory cards and tags, its card-name bytes show the type the reader detected: 00 01 MIFARE Classic 1K, 00 02 Classic 4K, 00 03 Ultralight, 00 26 Mini, F0 04 Topaz, F0 11 or F0 12 FeliCa. ISO 14443-4 cards carry historical bytes from the ATS (Type A) or ATQB (Type B) instead.
4. Read the UID with FF CA 00 00 00 and compare it with the byte order and number format your system stores, hex or decimal, before you enrol a batch.
5. Run each operation your application needs on the real card. For MIFARE Classic: load the key, authenticate, then read and write a test block, not a sector trailer. For DESFire or a Java Card: your application's own command sequence, including its longest command and response.
6. Unplug the reader, reconnect it and repeat, so you know your software reloads keys and settings on its own.

Ask for sample cards of the exact chip with the reader, so the test uses the stock you will order; standard samples are free.

Specifications listed by ACS

ACR122U, ACR1252U or ACR1552U

All three are ACS USB readers that PC/SC software opens as a CCID smart-card reader, and all three take the storage-card commands in the command table above. The ACR1552U lists more card types than the other two, and the three differ in their reader commands and in a few limits. Every figure in this table comes from ACS's own documents for that model, linked below it.

Sources: ACS's ACR1252U technical specification and API manual, the ACR1552U technical specification and reference manual, and the ACR122U API manual (PDF downloads).

Checks for PC/SC code moving off the ACR122U:

• The reader name. Software that selects the ACR122U by its reader name has to change. Each newer reader presents a contactless (PICC) reader and a SAM reader, and ACS's examples open "ACS ACR1252 1S CL Reader PICC 0" or "ACS ACR1552 1S CL Reader PICC 0". Select the PICC reader, not the SAM slot.
• Card commands carry over. Get Data (FF CA), Load Keys (FF 82, two volatile key slots), Authenticate (FF 86), Read and Update Binary (FF B0, FF D6), ACS's value-block commands (FF D7, FF B1) and the PC/SC 2.0 Part 3 transparent session (FF C2) are documented for both newer readers.
• Reader commands change. The ACR1252U keeps four of the ACR122U's FF 00 pseudo-APDUs as "ACR122U compatible commands": FF 00 40 (LED and buzzer), FF 00 48 (firmware version), FF 00 50 and FF 00 51 (polling settings). Otherwise both newer readers take escape commands that start E0 00, sent with SCardControl through the escape channel (next point); E0 00 00 18 00, for example, returns the firmware version. The beep on card detection, which the ACR122U controls with FF 00 52, is bit 3 of a behaviour byte that also sets the LEDs, on both: read the byte with E0 00 00 21 00 and write it back with E0 00 00 21 01 and bit 3 cleared, so the LED settings stay as they were. The ACR1252U keeps the new setting after power-off.
• Enable the escape channel for the new reader. On the ACR122U, reader commands can go as ordinary transmits while a card is on the reader, so existing software may never have used the escape channel. On the newer readers the E0 00 commands go only through it. With Microsoft's CCID driver on Windows, set EscapeCommandEnable under the new reader's own USB ID, not the ACR122U's VID_072F&PID_2200, or install ACS's driver, which ACS's ACR1552U manual says to install. On Linux, ACS's acsccid driver accepts SCARD_CTL_CODE(3500), and pcsc-lite's CCID driver (libccid) needs ifdDriverOptions 0x0001 and takes the escape as SCARD_CTL_CODE(1); on macOS, install ACS's acsccid driver.
• No PN532 commands. On the ACR122U, Direct Transmit (FF 00 00 00) passes commands to its PN532 controller, such as the antenna switch. Neither newer reader documents PN532 commands. They use the FF 00 00 00 header for FeliCa commands, and the ACR1552U also for ISO 14443-3 commands. The RF field is switched inside a PC/SC 2.0 Part 3 transparent session: FF C2 00 00 02 81 00 starts the session, FF C2 00 00 02 83 00 turns the field off and FF C2 00 00 02 84 00 on, and FF C2 00 00 02 82 00 ends the session. Automatic polling stays off until the session ends, and ACS's ACR1552U manual warns that a session used wrongly can stop the reader detecting cards. To save power, use the ACR1252U's automatic polling setting (E0 00 00 23), which can turn the antenna off when no card is found, or the ACR1552U's RF control command (E0 00 00 25).
• Card names in the ATR. MIFARE Classic 1K and 4K, Ultralight and Mini keep their card-name bytes (00 01, 00 02, 00 03, 00 26). FeliCa and Topaz change: the newer readers report 00 3B and 00 30, where the ACR122U reports F0 11 or F0 12 and F0 04. The ACR1552U polls for Topaz only after it is enabled with its Set PICC Polling Type command.
• Firmware. Record the firmware version of your sample. ACS lists a minimum firmware version for some ACR1552U commands, including the ISO 14443-3 and ISO 15693 pass-through commands.

What Proud Tek supplies

We supply the ACR122U, with the quantity confirmed per order, the two current ACS models compared above, and the cards, tags and stickers you test them with; our readers are listed under RFID readers. ACS makes all three readers and is the source of their drivers, firmware and manuals; our part is supply, and we neither write nor support application software. Tell us the card chip, the host operating system and the commands your software sends. Batches you would rather not encode yourself can go through our RFID encoding service: we read every encoded card back before it ships, each encoded order comes with a data report, and for numbered cards we add each card's UID beside its printed number in an Excel, CSV or XML file, which leaves the reader for spot checks.