IP to modem to serial interface

[Engineer_UA]

Hello all,
I'm trying to better understand communications circuits. When we have IP connection to a modem, which then has a serial connection(RS-232) to a meter. For the serial interface between a meter and a modem, can we have any protocol such as DNP, modbus, ION, etc? Does this protocol still utilize IP for data packet transmission or no?
Thanks,
EE

 

[Corsair]

Ethernet to Serial Conversion

Most manufacturers (AB, Schneider, even GE) started out with a serial protocol standard to use over RS-232 and RS-485. They then defined a protocol with a very similar data packet to use over Ethernet. The serial and Ethernet versions are not identical but a gateway box that is specific to that pair of protocols can be used to connect things. The most common example is Modbus TCP which is an Ethernet adaptation of Modbus RTU. Early AB Ethernet protocols were an adaptation of their serial DF1. GE had SNP and SNPX which were greatly modified to turn into their SRTP protocol. The key here is that the gateway box must understand both protocols.

There are general Ethernet to Serial gateways that pass characters through the Ethernet network and through the serial port. They are not specific to any particular protocol. You could not hit one of these gateways with Modbus TCP and have it go to serial RTU since the data packets are not completely identical. There are other complications like timing issues with these gateways. They may work well in some applications if the equipment on both ends is compatible.

All this is just some background information for your investigation.

 

[Engineer_UA]

Most manufacturers (AB, Schneider, even GE) started out with a serial protocol standard to use over RS-232 and RS-485. They then defined a protocol with a very similar data packet to use over Ethernet. The serial and Ethernet versions are not identical but a gateway box that is specific to that pair of protocols can be used to connect things. The most common example is Modbus TCP which is an Ethernet adaptation of Modbus RTU. Early AB Ethernet protocols were an adaptation of their serial DF1. GE had SNP and SNPX which were greatly modified to turn into their SRTP protocol. The key here is that the gateway box must understand both protocols.

There are general Ethernet to Serial gateways that pass characters through the Ethernet network and through the serial port. They are not specific to any particular protocol. You could not hit one of these gateways with Modbus TCP and have it go to serial RTU since the data packets are not completely identical. There are other complications like timing issues with these gateways. They may work well in some applications if the equipment on both ends is compatible.

All this is just some background information for your investigation.

Thank you! Is there a book you would recommend to obtain fundamental knowledge on this?

 

[JeffKiper]

Corsair did a great job of explaining this.

I typically explain it to people using a phone system to call someone. Here is my very crude breakdown.
You have the physical conduit that the call is transferred over. Old school analog, cellular, VoIP, a string between 2 tin cans. These would be serial 232, 422, 485, Ethernet, fiber....

Then you have the the protocol language. English, Spanish, French... Modbus RTU, ASCII, DF1, ETHERNET/IP, MODBUS TCP.....
So if you wanted to talk DF1 232 to DF1 232 and the hardware handles it natively you dont need anthing special. If you want to talk Modbus TCP over fiber to a wireless Ethernet/IP you will need a few adapters. You will need a fiber to wireless adaper. Then probably a wireless to copper adapter for the Ethernet /IP. Somewhere in the system you will need a translator (we call them protocol converters)

Just imagine making a call fron New York on a VoIP phone to a rural village in Mexico. The phone company will handle all the physical media conversion. You will still need to handle the language barrier. So an interpreter could change English to Spanish and visa versa.

Thats the best way I've found to explain protocol and media

 

[Corsair]

JeffKiper did a good job of introducing the idea of layers with data communications. So many times we've seen situations where we asked what protocol was used by a device and the answer was 'RS-232'.

I've never been convinced of the need to understand the definitions of each layer of the OSI model - maybe the college professor types are fascinated with that. I think the first step to learning how protocols work is to study the first PLC protocol - Modbus in it's RTC and TCP forms. Modbus is loved and hated - many of us have felt both ways several times.

The Modbus documentation is freely available. It may be the only protocol that is simple enough for a beginner to be able to wrap his or her head around. Other protocols are much more complex with routing, data types, discovery, and other features.

Spend some time studying Modbus even if you do not plan on using it. The Modbus organization has the spec. A majority of the function codes can be ignored. The PLC class outline at Corsairhmi.com has some information that may be a help.

Ethernet/IP is a tremendous protocol but not for beginners. Walk before you run.

- - -

From this thread you can understand that a gateway from Modbus TCP to Modbus RTU might be pretty easy to configure. A gateway from Ethernet/IP to Modbus RTU would probably require more complex configuration. You can buy equipment to translate from almost anything to almost anything else if you can tolerate the complication. Some knowledge of both protocols is essential.