Detail description about How Modbus protocol works and Information about Modbus TCP/IP stack. Description about how TCP/IP stack communicate between Server and Client from socket creation to socket close

1. 8/2/2014 1
Development of a Modbus RTU to Modbus TCP/IP
compiler on Netburner platform
Prepared by: Pratik Vyas

2. 8/2/2014 [Project Name] 2
Introduction
• An open source protocol – Modicon Inc.
• Dick Morley Innovator
• For Industrial Automation System
• Communicate over Master – Slave Technique
• Remotely access Machineries
• Less human effort require
• Modbus TCP/IP
• Modbus RTU

3. 8/2/2014 [Project Name]
Modbus TCP/IP Stack
3

4. 8/2/2014 [Project Name]
Modbus TCP/IP Stack
4

5. 8/2/2014 [Project Name] 5
• Creation of socket()
• int sockid= socket(family, type, protocol);
• sockid: socket descriptor
• family: integer, communication domain, PF_INET
• type: communication type like SOCK_STREAM &
SOCK_DGRAM
• protocol: specifies protocol, IPPROTO_TCP:: Indicates
that the TCP protocol is to be used
• socket call does not specify where data will be coming from,
nor where it will be going to –it just creates the interface!
Socket

6. 8/2/2014 [Project Name] 6
Modbus TCP/IP Stack

7. 8/2/2014 [Project Name]
• reserves a port for use by the socket
• int status = bind(sockid, &addrport, size);
• sockid: integer ID
• addrport: the (IP) address and port of the machine
 for TCP/IP server, internet address is usually set to
INADDR_ANY, i.e., chooses any incoming interface
• size: the size (in bytes) of the addrport
• status: upon failure -1 is returned
7
Bind

8. 8/2/2014 [Project Name]
int sockid;
struct sockaddr_in addrport;
sockid= socket(PF_INET, SOCK_STREAM, 0);
addrport.sin_family= AF_INET;
addrport.sin_port= htons(5100);
addrport.sin_addr.s_addr = htonl(INADDR_ANY);
if(bind(sockid, (struct sockaddr *) &addrport,
sizeof(addrport))!= -1)
{
…
}
8
Bind

9. 8/2/2014 [Project Name]
• bind can be skipped
• Stream socket: The OS finds a port each time the socket sends a
packet
9
Bind

10. 8/2/2014 [Project Name] 10
Modbus TCP/IP Stack

11. 8/2/2014 [Project Name]
• Instructs TCP protocol to listen for connections
• int status = listen(sockid, queueLimit);
• sockid:: integer, socket descriptor
• queuelen:: integer, # of active participants that can “wait” for a
connection
• status:: 0 if listening, -1 if error
• The listening socket is used by the server only as a way to get
new sockets
11
Listen

12. 8/2/2014 [Project Name] 12
Modbus TCP/IP Stack

13. 8/2/2014 [Project Name]
• The client establishes a connection with the server by calling
connect()
• int status = connect(sockid, &foreignAddr, addrlen);
– sockid:: integer, socket to be used in connection
– foreignAddr:: address of the participant
– addrlen:: integer, sizeof(name)
• connect() is blocking
13
Connect

14. 8/2/2014 [Project Name]
• The server gets a socket for an incoming client connection by
calling accept()
• int s = accept(sockid, &clientAddr, &addrLen);
– s:: integer, the new socket (used for data-transfer)
– sockid:: integer
– clientAddr:: address of the active participant
– addrLen:: sizeof(clientAddr)
• accept() waits for connection before returning
14
Accept

15. 8/2/2014 [Project Name] 15
Modbus TCP/IP Stack

16. 8/2/2014 [Project Name]
• int count = send(sockid, msg, msgLen, flags);
– msg:: message to be transmitted
– msgLen:: length of message (in bytes) to transmit
– flags:: special options, usually just 0
• int count = recv(sockid, recvBuf, bufLen, flags);
– recvBuf:: stores received bytes
– bufLen:: bytes received
– flags:: special options, usually just 0
• Calls are returns only after data is sent / received
16
Exchanging Data

17. 8/2/2014 [Project Name] 17
Close

18. 8/2/2014 [Project Name]
• When finished using a socket, the socket should be closed
• status= close(sockid);
– sockid:: socket being closed
– status:: 0 if successful, -1 if error
• Closing a socket closes a connection
18
Close

19. 8/2/2014 [Project Name]
User Interface
19

20. 8/2/2014 [Project Name]
Further Implementation
• After getting data from the client forward it on Modbus RTU
stack and RTU stack will send it to device.
• In reverse device will send information will get to the Modbus
RTU stack and it will return to back on Modbus TCP/IP stack.
20

21. 8/2/2014 [Project Name]
Thank you
21