Vending Machine/Cashless Device

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MDB/ICP protocol

The Multi-Drop Bus/Internal Communications Protocol is a voluntary standard for vending machine communication. This is a serial bus interface working at 9600baud in a master-slave arrangement, where the vending machine controller is the master.Each peripheral is assigned a unique address and command set.

MDB has support for the following devices:-

  • Coin Changer
  • Bill Validator
  • Cashless devices
  • Communications Gateway
  • Universal Satellite Device
  • Coin Hopper or Tube Dispenser

In order to operate the serial line as a bus with multiple devices attached MDB uses a 9bit serial over the traditional 8bit, 9n1 vs 8n1 The master Poll's each device for activity, all communication sessions are initiated by the master addressing a slave, slaves are only allowed to respond when addressed, this prevents bus collisions.

The master indicates an address byte/start of session by setting the 9th bit, the correct slave responds as needed and on sending it's last byte it also sets the 9th bit to indicated end of it's session (although the master will ACK this)

We've started build now on own home grown Cashless Device

The MDB spec already takes into account cashless devices, so we simply have to build a device that conforms to the spec.

Device Level

The specification defines 3 levels of cashless devices, with more more features the higher the level. The Vending Machine (VMC) also has a level, and a device and VMC can only communicate using the highest commonly supported level. As our VMC is only level 1, we only need to implement a level 1 cashless device.

So let's look at what a level 1 device needs to do. Here we will reference commands by their plain English names. For the technical details on these commands, please refer to the specification section 7. For details on how these commands are sent/received by the VMC, refer to secitons 2,3 and 4.


A (slightly simplified) diagram of the state machine, with commands sent from/to the VMC in capitals.

The cashless device is basically a state machine with 5 states (7 for level 2 and 3 machines). These states are:

  1. Inactive
  2. Disabled
  3. Enabled
  4. Session Idle
  5. Vend

When turned on, the cashlss device enters state 1. Almost all state transistions are initiated by the VMC, with the exception of 3 -> 4, which is initiated by the cashless device on a valid card read.

If the cashless device receives a command it cannot act upon in its current state, or receives an unexpected command whilst in an uninterruptable state it must ACK(nowledge) the command, then on the next POLL from the VMC, issue a COMMAND OUT OF SEQUENCE response.

The VMC will then respond with a RESET command, putting the reader back to state 1.

Uninterruptable States

During certain command sequences, the VMC should not issue any additional commands to the cashless device. These are the Uninteruptable States. If the VMC does issue any additonal commands during one of thes states, the reader should responds with COMMAND OUT OF SEQUENCE, as noted above.

These sequences for a level 1 device are:

VMC Command Cashless Device Response Result

The highlighted rows are all possible sequences in the vend state (state 5), so this state is completely uninterruptable.


Where a response is ACK, this can also be NAK (Not Acknowledged).

You will notice that some responses appear in the table twice. This is because the cashless device can respond ACK to a request for data, sending the data in response to a later POLL.

Command Sub-command / Data Response Req? Spec Page Num Implemented?
Reset (none) ACK Required 102 Yes
Setup Config Data Reader Config Data Required 103 Yes
Max/Min Prices ACK Required 106 Yes
Poll (none) Just Reset Required 108 Yes
Reader Config Data Required 108 No
Display Request Our VMC doesn't support this! 108 No
Begin Session Required 109 No
Session Cancel Request Required 112 No
Vend Approved Required 112 + 123 No
Vend Denied Required 113 + 123 No
End Session Required 113 + 126 No
Cancelled Required 113 + 133 No
Peripheral ID Required 113 No
Malfunction / Error Required 115 No
Command Out of Sequence Required 116 No
Diagnostic Response Required 122 + 147 No
Vend Vend Request Vend Approved Required 123 No
Vend Denied Required 123 No
Vend Cancel Vend Denied Required 125 No
Vend Success ACK Required 125 No
Vend Failure ACK Required 126 No
Session Complete End Session Required 126 No
Cash Sale ACK ?? 127 No
Reader Reader Disable ACK Required 132 Yes
Reader Enable ACK Required 132 Yes
Reader Cancel Cancelled Required 133 No
Expansion Request ID Peripheral ID Required 138 No
Diagnostics Diagnostic Response Optional? 147 No

Command Sequences

  • All sequences are shown from the point of view of the cashless device.
  • CHK indicate the calculated CHK byte
  • * indicates the mode bit is set.

Start Up / Reset

The only difference between these two sequences is the beginning. The start up sequence begins when the cashless device receives an address command from the VMC, the reset sequence when the cashless device receives a RESET command.

The only difference between these commands is that the mode bit is set. (--'RepRap' Matt 20:12, 16 August 2011 (BST) this is incorrect)

Command Received Command Sent Data Hex Current State
For Startup only
Address {none} 10H* CHK X
ACK (none) 00H* X
For Reset only
RESET (none) 10H* CHK X
ACK (none) 00H* X
POLL (none) 12H* CHK X
JUST RESET (none) 00H CHK* X
For Both
SETUP CONFIG DATA feature level, columns on display, rows on display, display info 11H* 00H 01H 00H 00H 00H CHK X
READER CONFIG DATA feature level, country code high, country code low, scale factor, decimal places, app max response time, misc 01H 01H 18H 26H 05H 02H 0AH 00H CHK* X

Not true!