ACS ACR31 User Manual

ACR31 Swipe

Card Reader

Reference Manual V1.00

Subject to change without prior notice [e-mail]

www.acs.com.hk

Table of Contents

1.0. Introduction ............................................................................................................. 3

2.0. Features ................................................................................................................... 4

3.0. System Block Design .............................................................................................. 5

4.0. Hardware Design ..................................................................................................... 6

4.1. Battery .................................................................................................................................... 6

4.2. Audio Channel ....................................................................................................................... 6

4.2.1. Communication Parameters ......................................................................................... 6

4.3. Magnetic Stripe Card Interface .............................................................................................. 6

5.0. Communication Protocol ........................................................................................ 7

5.1. Audio Channel Message ........................................................................................................ 7

6.0. Commands ............................................................................................................... 8

6.1. SPH_to_RDR_Reset ............................................................................................................. 8

6.2. SPH_to_RDR_Sleep ............................................................................................................. 9

6.3. SPH_to_RDR_FirmwareVersion ......................................................................................... 10

6.4. SPH_to_RDR_GetStatus ..................................................................................................... 11

6.5. SPH_to_RDR_SetSleepTimeout ......................................................................................... 12

6.6. RDR_to_SPH_GetStatus ..................................................................................................... 13

6.7. RDR_to_SPH_FirmwareVersion ......................................................................................... 14

6.8. RDR_to_SPH_TrackData .................................................................................................... 15

6.9. RDR_to_SPH_ACK ............................................................................................................. 17

Appendix A. Track Data Error Code .............................................................................. 18

Appendix B. System Error Codes ................................................................................. 19

List of Figures

Figure 1 : System Block Diagram ........................................................................................................... 5

List of Tables

Table 1 : 3.5mm Audio Socket Wiring .................................................................................................... 6

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1.0. Introduction

The ACR31 Swipe Card Reader serves as an interface for the communication between a mobile
device and a magnetic stripe card. Different types of magnetic cards have different commands and
communication protocols, and the ACR31 establishes a uniform interface from the mobile device to
the magnetic card.

The ACR31 is connected to the mobile device through a 3.5-mm audio jack interface. Through this,
the ACR31 will read information from the magnetic cards through the decoder on the reader which will
be sent to the mobile device, such as a smartphone or tablet. Furthermore, as a way to enhance
security, the card information is encrypted using the AES-128 encryption algorithm before it will be
sent to the backend server.

This document describes the hardware and software design of the ACR31 and the list of commands it
uses to communicate with the mobile device.

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2.0. Features

• 3.5-mm Audio Jack Interface

• Powered by a CR2016 battery

• Reads up to two tracks of card data

• Capable of bi-directional reading

• Supports AES-128 encryption algorithm

• Magnetic Stripe Card Reader:

o Supports ISO 7810/7811 magnetic cards

o Supports Hi-coercivity and Low-coercivity magnetic cards

o Supports JIS1 and JIS2

• Supports Android™ 2.3 and above*

• Supports iOS 5.0 and above*

• Compliant with the following standards:

o CE

o FCC

o VCCI

o RoHS

o REACH

*Note: Visit www.acs.com.hk

for the list of supported devices.

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ACR31

Decoder IC

3.0. System Block Design

The system block design of ACR31 is depicted below:

Magnetic Head

Magnetic Stripe

ISO7810/7811

Power

Management

Audio Jack

Mobile Device

MCU

Battery

Figure 1: System Block Diagram

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4.0. Hardware Design

4.1. Battery

ACR31 is using a CR2016 battery which has a capacity of 90 mAh.

4.2. Audio Channel

4.2.1. Communication Parameters

ACR31 is connected to a mobile device through Audio Channel.

Pin Signal Function

1 Left Transmit the data to ACR31

2 Right Reset signal to ACR31

3 GND GND

4 MIC Transmit the data to smart phone

Table 1: 3.5mm Audio Socket Wiring

4.3. Magnetic Stripe Card Interface

ACR31 can read any magnetic stripe card that conforms to ISO 7810/7811 standards. ISO 7810
specifies the physical characteristics of the card, while ISO 7811 specifies the recording technique
used in identification cards.

High-coercivity (HiCo) magnetic stripes are typically black in color and are encoded with a stronger
magnetic field (2750 Oersted). This makes HiCo cards more durable because the data encoded on
the stripes are less likely to be unintentionally erased when exposed to an outside magnetic field.
When swiped across the magnetic head, HiCo magnetic stripes can induce larger signal pulses and
are more easily being detected and decoded.

Low-coercivity (LoCo) magnetic stripes are generally brown in color and are encoded with lower
magnetic field intensity (300 Oersted). They will induce small signal pulses compared to HiCo cards
when swiped across the magnetic head. As a result, the S/N (signal-to-noise) ratio is relatively low
and they are more vulnerable to noise interference. A more sophisticated hardware support and signal
processing algorithm are needed to decode the signal correctly.

Since the magnetic fields of HiCo and LoCo cards are different, a magnetic stripe decoder IC with
automatic gain control can be used in the design to cater these two types of cards.

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5.0. Communication Protocol

ACR31 is a slave device and almost all operations are initiated by the mobile device. The mobile
device that sends the command is carried out in the form of successive command request-response
exchange. Additionally, the new request message should wait until previous response message has
been received (except interrupted by the RDR_to_SPH_TrackData interrupt message).

ACR31 will communicate with a mobile device through its audio jack interface. The communication
channel is bi-directional, with the reader sending data to the mobile device through the MIC pin of the
audio jack while the mobile device sending commands to the reader through the Left-Channel of the
audio jack.

While it’s not operating, the ACR31 will remain in deep sleep mode. Upon receiving a wake up signal
from the mobile device through the Right-Channel of the audio jack, the ACR31 will wake up. ACR31
will then wait for the swipe of the magnetic stripe card or any command message sent by the mobile
device within a timeout limit. After successfully obtaining the data from the swiped card, the ACR31
will perform AES-128 encryption on the received card data and send back the encrypted data to the
mobile device in communication. If the reader fails to obtain a card swipe or command message
within the timeout limit, the ACR31 will go back to deep sleep mode to save battery power.

Before the communication protocol between the ACR31 and the mobile device employ a direct signal
feeding, the signal received from the ACR31 will be passing through a DC offset cancellation filter.
The data to be transmitted will be encoded using the Manchester coding scheme (conforms to IEEE

802.3), with the clock frequency used in the Manchester coding scheme being set at 2.756 kHz. Since
the data transmission speed in the Manchester coding scheme always matches the clock speed, a
maximum baud rate of around 2.756 kbps could be achieved.

The signal interpretation on the mobile device and ACR31 is based on sampling the corresponding
input waveforms. The sampling frequency should be at least double the clock frequency (Nyquist
rate) used in the Manchester coding scheme. After sampling the signals, the data encoded in the
signals could be received by determining the logical zero-crossing time.

5.1. Audio Channel Message

All messages between ACR31 and the mobile device consist of a header containing a frame start byte
(1 byte), a length field (2 bytes) and a message type indicator (1 byte). There is 2-byte CRC at the
end of each message for error detection. The card data will be encrypted using AES-128 encryption
algorithm.

Below list all the messages that would appear between the communications of ACR31 and the mobile
device:

Send-Out Message Code Send-In Message Code

SPH_to_RDR_Reset N/A RDR_to_SPH_GetStatus 80h

SPH_to_RDR_Sleep 63h RDR_to_SPH_FirmwareVersion 81h

SPH_to_RDR_FirmwareVersion 6Fh RDR_to_SPH_ACK 82h

SPH_to_RDR_GetStatus 6Eh

SPH_to_RDR_SetSleepTimeout 6Ch

RDR_to SPH_TrackData 50h

Interrupt-In Message Code

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6.0. Commands

6.1. SPH_to_RDR_Reset

This reset command will not send any message to ACR31. Instead, the mobile device will generate
several pulses in the right channel of the audio jack that will trigger the ACR31 to perform a hardware
reset. This is the only way to resume the normal operation of ACR31 after it has entered the sleep
mode.

No response will be generated for this command.

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6.2. SPH_to_RDR_Sleep

This command is used to force the ACR31 to enter sleep mode immediately upon receiving this
command.

Offset Field Size Value Description Encrypted

0

1

3

4

The response to this message is RDR_to_SPH_ACK.

bStartByte

wLength

bMessageType

wCheckSum

1 23h Message start byte.

2 0003h

1 63h

2

Number of extra bytes starting
from the next field for this
message.

CRC16 checksum for all the
preceding fields.

No

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6.3. SPH_to_RDR_FirmwareVersion

This command is used to retrieve the firmware version string from ACR31.

Offset Field Size Value Description Encrypted

0

1

3

4

When there is no error, the response to this message is RDR_to_SPH_FirmwareVersion.

Otherwise, the response is RDR_to_SPH_ACK which will specify the corresponding error.

bStartByte

wLength

bMessageType

wCheckSum

1 23h Message start byte.

Number of extra bytes starting

2 0003h

1 6Fh

2

from the next field for this
message.

CRC16 checksum for all the
preceding fields.

No

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6.4. SPH_to_RDR_GetStatus

This command is used to request ACR31 to give a feedback of the current device status to the mobile
device.

Offset Field Size Value Description Encrypted

0

1

3

4

When there is no error, the response to this message is RDR_to_SPH_GetStatus.

Otherwise, the response is RDR_to_SPH_ACK which will specify the corresponding error.

bStartByte

wLength

bMessageType

wCheckSum

1 23h Message start byte.

Number of extra bytes starting

2 0003h

1 6Eh

2

from the next field for this
message.

CRC16 checksum for all the
preceding fields.

No

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6.5. SPH_to_RDR_SetSleepTimeout

This command is used to request ACR31 to change the sleep timeout value to the one specified.

Offset Field Size Value Description Encrypted

0

1

3

4

4

The response to this message is RDR_to_SPH_ACK.

bStartByte

wLength

bMessageType

bTimeout

wCheckSum

1 23h Message start byte.

2 0004h

1 6Ch

1

2

Number of extra bytes starting
from the next field for this
message.

Sleep timeout value in seconds.
This value should be in the
range of 4 – 20. Other values
are not allowed.

CRC16 checksum for all the
preceding fields.

No

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6.6. RDR_to_SPH_GetStatus

This message is sent by ACR31 in response to the command message SPH_to_RDR_GetStatus.

Offset Field Size Value Description Encrypted

0

1

3

4

5

bStartByte

wLength

bMessageType

bBatteryLevel

bSleepTimeout

1 23h Message start byte.

Number of extra bytes starting

2 0006h

1 80h

1

1

from the next field for this
message.

00h – Battery Level ≥ 3.0V

01h – 2.9 ≤ Battery Level < 3.0 V

02h – 2.8 ≤ Battery Level < 2.9 V

03h – 2.7 ≤ Battery Level < 2.8 V

04h – 2.6 ≤ Battery Level < 2.7 V

05h – 2.5 ≤ Battery Level < 2.6 V

06h – 2.4 ≤ Battery Level < 2.5 V

07h – 2.3 ≤ Battery Level < 2.4 V

08h – Battery Level < 2.3 V

Active sleep timeout value used
in the reader.

No

6

7

bRFU

wCheckSum

1 00h Reserve for future use.

2

CRC16 checksum for all the
preceding fields.

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6.7. RDR_to_SPH_FirmwareVersion

This message is sent by ACR31 in response to the command message
SPH_to_RDR_FirmwareVersion.

Offset Field Size Value Description Encrypted

0

1

3

4

16

bStartByte

wLength

bMessageType

abFirmwareVersion

wCheckSum

1 23h Message start byte.

Number of extra bytes starting

2 000Fh

1 81h

12 Firmware version string.

2

from the next field for this
message.

CRC16 checksum for all the
preceding fields.

No

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6.8. RDR_to_SPH_TrackData

This message is sent by ACR31 when it detects the swiping of card and transfers Track 1 and Track 2
data to the mobile device.

There is no need for the mobile device to send any command message to ACR31 to retrieve the card
data. This message is sent automatically from ACR31 to the mobile device once a card swipe action
is performed and thus is known as an interrupt message.

Offset Field Size Value Description Encrypted

0

1

3

4

83

123

124

125

bStartByte

wLength

bMessageType

abTrack1Data

abTrack2Data

bFieldSeparator

bTrack1DataLength

bTrack2DataLength

1 23h Message start byte.

Number of extra bytes starting

2

1 50h

79

40

1 0Dh

1

1

from the next field for this
message.

Magnetic stripe card track 1
data.

Magnetic stripe card track 2
data.

Effective data length or error
code.

If the MSB is set, It is an error
code. Otherwise, it is data
length.

Effective data length or error
code.

If the MSB is set, It is an error
code. Otherwise, it is data
length.

No

Yes

126

130

132

133

134

bLowVoltageDetector

abRFU

wChecksum

bFieldSeparator

bTrack1Error

4 Reserved for future use.

2

1 0Dh

1

1

CRC16 checksum for data
starting from offset 4 to 129.

Detect the power of Battery
00h – Not enough power
01h – Enough power

Track 1 error register.

If the MSB is set, It is an error
code. Otherwise, it is data
length.

No

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Offset Field Size Value Description Encrypted

Track 2 error register.

135

bTrack2Error

1

If the MSB is set, It is an error
code. Otherwise, it is error-free.

136

wCheckSum

2

CRC16 checksum for all the
preceding fields.

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6.9. RDR_to_SPH_ACK

This acknowledgement message is sent by ACR31 to the mobile device specifying the acceptance of
certain command messages. This message also includes the error code whenever appropriate.

Offset Field Size Value Description Encrypted

0

1

3

4

5

bStartByte

wLength

bMessageType

bErrorCode

wCheckSum

1 23h Message start byte.

Number of extra bytes starting

2 0004h

1 82h

1

2

from the next field for this
message.

Specify the error code for the
previously processed command
message.

The possible error codes are
listed in Appendix B.

CRC16 checksum for all the
preceding fields.

No

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Appendix A. Track Data Error Code

Bit 7

Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1

MSB

MSB 0 0 0 0 LRC error End Sentinel error Start Sentinel error

Bits 7 to 1 are error codes.

Error-free = 0

Bit 0

LSB

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Appendix B. System Error Codes

The following table lists all the system error codes and their corresponding description for ACR31.

Error Code Status

00h ERROR_SUCCESS

FFh ERROR_INVALID_CMD

FEh ERROR_INVALID_PARAM

FDh ERROR_INVALID_CHECKSUM

FCh RROR_INVALID_STARTBYTE

FBh ERROR_UNKNOWN

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