Discussion in "New Ideas regarding projects" started by    esm    May 12, 2012.
Sat May 12 2012, 03:35 PM
#1
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).



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

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