UART over ISO-11898 signalling ??? RS-485 vs. ISO-11898 discussion

Discussion in 'The Projects Forum' started by esm., May 12, 2012.

  1. esm.

    Thread Starter Member

    Jan 20, 2012
    30
    0
    Hi.

    ISO-11898 specifies the data link layer (DLL) and physical signalling of the controller area network (CAN protocol)
    I want to discuss a little about advantages and disavantages between RS-485 and ISO-11898 electrical standards, for a half-duplex communication bus.
    Both ISO-11898 and RS-485 use differential signalling.

    Considering the following:

    - A bus containing a maximum of 60 nodes, operating at a baudrate between 20 and 50Kbps, with a maximum lenght of 1000 meters
    - I don't want to use a MCU with integrated CAN module, I want to use a simple UART module of this MCU, but operating at ISO-11898 signalling levels, and I'm thinking to implement MODBUS RTU protocol over the UART to gain confiability and use a public protocol.
    - In all nodes, there will be galvanic insulation between the MCU and the ISO-11898 bus. I'm planning to use a DC-DC converter to insulate the power supplies and the ADuM1201 IC to insulate TX and RX pins of MCP2551 from the MCU
    - Master-slave bus (there will be only one master on the bus). If I need to change this bus to a multi-master type, in future, I will need to make a customized MODBUS protocol, and I think, in hardware questions, ISO-11898 is more suitable than RS-485 for a multimaster bus.


    So, I did a fast comparative between MAX487 (RS-485) and MCP2551 (ISO-11898) transceivers. OBS: maybe some important parameters to be compared are missing.
    My conclusion was that there are more advantages on using ISO-11898 instead of RS-485, in this case.

    Which physical signalling you would choose in such case ??? RS-485 or ISO-11898 ???


    MCP2551-I/SN (MICROCHIP) ---> ISO-11898 Transceiver, SMD SOIC-8
    - DIGIKEY - Quantity Available = 44,788
    - DIGIKEY - Cost of 25 units = USD 21.25
    - Temperature: -40°C to +85°C
    - Up to 112 nodes attached to the bus
    - Supply range: 4.5 V ~ 5.5 V
    - No possibility of shor-circuit on the bus when two or more nodes wants to transmit at the same time, due to dominat and recessive states of the bus
    - There is no need to control the direction on data (transmit, receive). The tranceiver is always reading the bus and have a push-pull RX output ('0' OR '1' only). No need of pull-up resistor at RX pin
    - DOUBT: DOES A ISO-11898 BUS NEED A FAIL-SAFE BIASING RESISTORS, LIKE IN A RS-485 BUS ???. I THINK NOT, BUT DON'T HAVE SURE.
    - PESD1CAN IC, SMD SOT-23 - Protection TVS diode for ISO-11898 lines. Cost of 25 units at DIGIKEY: USD 7.94


    MAX487ECSA+ (MAXIM) ---> RS-485 Transceiver, SMD SOIC-8
    - DIGIKEY - Quantity Available = 1,461
    - DIGIKEY - Cost of 25 units = USD 59.85
    - Temperature: -40°C to +85°C
    - Up to 128 nodes attached to the bus
    - Supply range: 4.75 V ~ 5.25V
    - Possibility of shor-circuit on the bus when two or more nodes wants to transmit at the same time
    - Need to control the direction on data (transmit, receive). In insolated sytems, higher costs due to an extra optocoupler. Need of pull-up resistor at RO pin
    - A RS-485 bus need fail-safe biasing resistors, to assure a voltage higher than 200mV between A and B on all the nodes, when there is no nodes transmiting on the bus.
    - SM712 IC, SMD SOT-23 - Protection TVS diode for RS-485 lines. Cost of 25 units at DIGIKEY: USD 21.00



    Another doubts about ISO-11898 are:

    1) DOES A ISO-11898 BUS NEED A FAIL-SAFE BIASING RESISTORS, LIKE IN A RS-485 BUS?

    2) By using ISO-11898 tranceivers like MCP2551, is it possible to have higher bus lengths? Example: a 1000 meter length bus, operating around 20 to 50kbps?

    3) These kind of cable is suited for a ISO-11898 bus? It has 120 ohms characteristic impedance and they are also used in RS485 buses. They are also known as Profibus cable (just seach for it on google images).

    [​IMG]

    Somebody with good knowledge on RS-485 and ISO-11898 ???
    Thanks.
     
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