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).
**broken link removed**
Somebody with good knowledge on RS-485 and ISO-11898 ???
Thanks.
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).
**broken link removed**
Somebody with good knowledge on RS-485 and ISO-11898 ???
Thanks.