RS485 protocol

Last post reminded me to write about library I have created for the wired network. It is called simply LibRS485, or NilRS485 adopted to work in NilRTOS. The library defines packet protocols on RS485 drivers. I use ST3485EB a 3.3 V powered, 15 kV ESD protected and up to 12 Mbps RS-485 half duplex transceiver. Inexpensive SO8 chip that has all that is needed already inside. As it is half duplex we need one extra pin to tell the transceiver if we are receiving or transmitting. The transmission itself require one twisted pair of wire and is able to push the data to distances over 1km with reasonable speed. The nice thing about RS485 it uses a differential balanced line and resists electromagnetic interference from motors and other equipment, more can be found on wiki.

LibRS485 uses a 9bit protocol and Multi-processor Communication mode defined in ATmega. This allows the MCU to do other thing without looking on communication unless there is a address frame. Basically i work like this:

1. All nodes are in Multi-processor Communication mode (MPCM)
2. One of the node sends an address frame, and all receive and read this frame.
3. All nodes reads the address frame and determine if it has been selected. If so, it clears the MPCM, otherwise it waits for the next address frame and
   keeps the MPCM setting.
4. The addressed node will receive all data frames until a new address frame is received.
5. The other nodes which still have the MPCM bit set, will ignore the data frames.
6. When the last data frame is received by the addressed node, the addressed MCU sets the MPCM and waits for a new address frame. The process then
   repeats from 2.

Library also defines a packet structure like this:

 -------------~~~~~~~~~~~~~~~~~~~~~~
 - A - P - X -  D0 - D63 ~ S0 ~ S1 ~
 -------------~~~~~~~~~~~~~~~~~~~~~~
  
  - Required fields
  ~ Data and signature fields
  A Address - 4 bits form and 4 bits to address.
    0  is master, but not really as any node can drive communication
    15 is broadcast to all
    14 addresses left for other devices
  
  P Packet definition - 2 bits type and 6 bits length.
    Flag bit configuration:
    FLAG_ACK 3
    FLAG_NAK 2
    FLAG_CMD 1
    FLAG_DTA 0
     
    Types: CMD - Command - user defined general commands value 0 - 63
           DTA - Data - user data length D0 - D63 and CRC S0, S1
           ACK, NAK - flags for data, using signature of data packet
           
  X XOR of A and P, for basic transfer safety.
  
  D0 - D63 user data.     
                       
  S0 ~ S0  CRC for user data or signature for ACK, NAK.   

So basically there are two types of packets. Command packet that has only 3 bytes (APX) and deliver a command(number 0-63) to node(s). It is used for example to issue a call for registration. Or data packet  that has 6-69 bytes (APX D0~D63 S0S1). This deliver any data to node(s). The nodes are addressed by its address 0-14, which select any individual node. Or address 15 that is broadcast to all nodes.

Web interface - authentication

Authentication units have their own configuration page as shown on the right. On the top, again, is a summary of authentication units connected to system. The units can be connected by wire(RS485) or wireless(RFM12B). It is shown in the address field. "W" stands for wired and "R" for Radio. Address range is the decision maker here. Units in range 1-14 are wired and 17-143 are connected wireless. Address and type of the authentication unit is defined in remote unit itself. The few options to set is turn the unit On or Off, other is to attach them to certain group.

Remote units are dynamically added to the system at any time by process called registration. This process is activated by service thread few seconds after the system starts, or when you power up any remote units it will issue call for registration. If the unit is present already the system will re-reregister it with new parameters. The actions taken are presented in the log. You can for example see following messages:

If you miss any unit that should be there you can push "Reregister" button and the system will issue registration call.

Going little more in detail, the all the settings are permanently stored in remote units in-chip EEPROM. By default the units register with their defaults, that is not enabled and belonging to group 16. And you have to set them on first. The other settings like functions they provide is dependent on the hardware and is coded in the registration message they sent to main board. Any remote unit can provide more then one function, as you can see on picture above. Unit 1 is iButton authentication unit and also provide ambient temperature.

Web interface - phone numbers

Web interface also include phone numbers setup. There is now space for 8 numbers. On the top you can see the overview in a table. Bellow there is selection button to chose to work with entries. Phone can be enabled or disabled, so you can temporary take anyone out. You can create unique name for every phone number for convenience. And it will appear by this name on other configuration pages. If you set "Global" option to yes the number will be informed about any event. As opposed to "Global No" the number will be informed only about group specified events. This page only specifies the setting for individual numbers, selecting which event will be passed is done in global configuration page.

Web interface - keys

Another configuration page is about iButton keys. You can see the page on the right. Settings here are rather simple on the top you can see the table of assigned keys. There are name and and 8 byte address of iButton key. There is currently space to store up to 8 keys, but can be expanded. Bellow that is a pull down selection to work with single key.  There is not much to say about that except of last key value that is set to 55's. This value hold the last unauthorized key address. That can be used either to see address of key that is not yours. Or better when you have new key, you can just touch authorization unit. The system recognize unknown key, issue a warning message in the system log, and store the address value here. Next thing is to copy paste the value to some empty slot and give it a name, and press "Save all" to update the EEPROM. Well there is also laser printed address on every iButton, but in reverse order and it is bit tiny.

The system uses just ROM address of iButton, that mean you can use any iButton as a key even temperature probes and such. Now this bring another thought about security. First I consider iButton secure, but they can be read and copied, so can be RFID tags. Also touching probe with iButton is more convenient than to type 16 hex digit on keypad. But it can be surely improved. For example if you stay with iButton there is NVRAM version like DS1992. It gives you 1Kb of non volatile RAM you can use for floating code that can be generated every time you touch authorization unit. This makes coping much more complicated.

There is also possibility here to use other means of authorization, like NFC found on many phones. Or a simple keypad, but I have small children and prefer ROM iButton for its price and durability. Maxim states "Thanks to sealed stainless steel, you can drop it, step on it, or scratch it. The iButton is wear-tested for 10-year durability and a million hot contacts."