MDB Communication Protocal

Introduction

This document outlines a serial communication protocol between the application running on Q3v and MDB Vending machines. The Q3v low-level hardware forwards MDB requests from the vending machine to the Android application as serial data. When the application processes all request serial data, the Q3v POS can be regarded as an MDB cashless slave device. Additionally, the application can request configuration commands, such as "get firmware version," making the protocol bi-directional. A mode byte indicates the direction of communication.

Serial Protocol Packet Definition

Serial Port Parameters

Baud rate 115200
8bit, 1 stop bit, and no parity

Packet Format

Start code	Length	mode	Data	Checksum	End code

Start code size 1 byte, always be 0x09
Length size: 1byte, the number bytes of mode, data, and checksum.
Mode size: 1, 0x00 means a master request packet, 0x01 means a slave response packet, other value is prohibited.
Data size: n bytes. The data could be raw MDB commands, such as SETUP, VEND(please refer the <Multi-Drop Bus Protocol V4.3>for detials).

Or the MIB control commands, such as GET VERSION, SET PARAMETER.

Checksum size: 1byte, using LRC algorithm, input data were "mode, data"

Specifics

MDB Forwarding Mechanism

The Q3v forwards all MDB cashless device commands except the POLL command. The request and response procedure can be described in the following steps:

VMC sends MDB request to Q3v.
Q3v receives the data, removes the 9th MDB mode bit, fills it into the serial packet data part, and sends the serial packet to the application.
The application listens to the serial port, receives step 2 data, unwraps it to MDB request data, handles it, wraps its MDB response into a serial packet, and sends it to Q3v.
Q3v receives step 3 data, unwraps it to MDB response data, adds the MDB mode bit, and forwards it to the MDB master.
MDB master handles the MDB slave response.

Taking the RESET for example

VMC -> q3v: 0x110 0x10
q3v -> app: 0x09 0x04 0x00 0x10 0x10 0xE0 0x0D
app -> q3v: 0x09 0x04 0x01 0x00 0x00 0xFF 0x0D
q3v -> VMC: 0x00 0x100
VMC -> q3v: 0x00

Application Notes

Focus on steps 2 and 3 listed above.
The application is responsible for handling all cashless commands except the POLL command.
When the cashless reader is ready for a transaction, it initiates a begin session command to notify the VMC.
MDB raw data includes its checksum byte (distinct from the serial frame checksum).

A Typical MDB Transaction Flow

MDB transacrion flow

Protocol Command Definition

Common MDB Forwarding Command

Common MDB Forwarding Command Description: This command forwards raw MDB commands (including MDB CHK), with the MDB mode bit removed.

Mode	Data
Request 00H	VMC Request Command (Reset,Setup,Reader,Expansion,Vend)

Response packet

Mode	Data
Response 01H	Peripheral Response

Example:
q3v -> app 09 04 00 10 10 E0 0D

q3v <- app 09 04 01 00 00 FF 0D.

Begin Session Command

Begin Session Command Description: When the application is ready for a transaction, it issues a begin session command to inform the VMC master. This marks the beginning of a transaction. Balance amount is 2 bytes; if balance does not exist, fill with 0xFF. This command follows MDB spec definition but is initiated by the app.

Mode	Data
Request 00H	Begin Session (03H) + Funds Available + MDB Checksum

Response packet

Mode	Data
Response 01H	ACK 00H

Example:
q3v <- app 09 06 00 03 00 64 67 32 0d

q3v -> app 09 03 01 00 ff 0d

Session Cancel Request(0x04)

Session Cancel Request (0x04) Description: The application can end a payment session by issuing a session cancel request command. The POS acts as the master during this command.

Mode	Data
Request 00H	Cancel Session Req(04H) + Funds Available + Checksum

Response packet

Mode	Data
Response 01H	ACK 00H

Demo

Source code of terminal APP

APK

Source code of PC program

MDBForCoffeeDemo usage