Python TCP/IP Communication

Question

This is my python modbus tcp communication code and it wait at the this line and than stopping for the connection where is my fault:

sock.connect((TCP_IP, TCP_PORT))

(if i use slave program also not working) At the my client side i am using this code:

TCP_IP='10.0.2.15'
TCP_PORT=502
BUFFER_SIZE=39
sock=socket.socket(socket.AF_INET,socket.SOCK_STREAM)
sock.connect((TCP_IP,TCP_PORT))

This is the master side:

# Create a TCP/IP socket
TCP_IP = '10.0.2.2'
TCP_PORT = 502
BUFFER_SIZE = 39
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect((TCP_IP, TCP_PORT))
try:
 unitId = 16 # Plug Socket11
 functionCode = 3 # Write coil
 print("\nSwitching Plug ON...")
 coilId = 1
 req = struct.pack('12B', 0x00, 0x01, 0x00, 0x00, 0x00, 0x06, int(unitId), 0x03, 0xff, 0xc0, 0x00,
 0x00)
 sock.send(req)
 print("TX: (%s)" % req)
 rec = sock.recv(BUFFER_SIZE)
 print("RX: (%s)" % rec)
 time.sleep(2)
 print("\nSwitching Plug OFF...")
 coilId = 2
 req = struct.pack('12B', 0x00, 0x01, 0x00, 0x00, 0x00, 0x06, int(unitId), 0x03, 0xff, 0xc0, 0x00,
 0x00)
 sock.send(req)
 print("TX: (%s)" % req)
 rec = sock.recv(BUFFER_SIZE)
 print("RX: (%s)" % rec)
 time.sleep(2)
finally:
 print('\nCLOSING SOCKET')
 sock.close()

• 001: What's your question?
• CKocar: where is my fault ?
• Ajax1234: How specifically is the program not working? What is the error that you are receiving?
• CKocar: i have no error i said program waiting this line and stopping without connection sock.connect((TCP_IP, TCP_PORT))

Answer 1

I think your problem is the IP address: 10.0.2.2 as stated here [Connection to LocalHost/10.0.2.2 from Android Emulator timed out. You can replace '10.0.2.2' with 'localhost' or try to find your IPv4 address.

To do so type ifconfig in your command prompt if you use Linux or type ipconfig in windows and search for the IPv4 address.

I used a simple client-server example to run your code and replaced '10.0.2.2' with 'localhost' and everything went fine.

server side:

import socket
import struct
import time
TCP_IP = 'localhost' 
TCP_PORT = 502
BUFFER_SIZE = 39
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind((TCP_IP, TCP_PORT))
s.listen(1)
sock, addr = s.accept()
print 'Connected by', addr
try:
 unitId = 16 # Plug Socket11
 functionCode = 3 # Write coil
 print("\nSwitching Plug ON...")
 coilId = 1
 req = struct.pack('12B', 0x00, 0x01, 0x00, 0x00, 0x00, 0x06, 
 int(unitId), 0x03, 0xff, 0xc0, 0x00,
 0x00)
 while 1:
 sock.send(req)
 print("TX: (%s)" % repr(req))
 rec = sock.recv(BUFFER_SIZE)
 print("RX: (%s)" % repr(rec))
 time.sleep(2)
 break
 print("\nSwitching Plug OFF...")
 coilId = 2
 req = struct.pack('12B', 0x00, 0x01, 0x00, 0x00, 0x00, 0x06, 
 int(unitId), 
 0x03, 0xff, 0xc0, 0x00,
 0x00)
 while 1:
 sock.send(req)
 print("TX: (%s)" % repr(req))
 rec = sock.recv(BUFFER_SIZE)
 print("RX: (%s)" % repr(rec))
 time.sleep(2)
 break
finally:
 sock.close()

client side:

import socket
TCP_IP = 'localhost' 
TCP_PORT = 502 
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect((TCP_IP, TCP_PORT))
data = sock.recv(1024)
print repr(data)
while 1:
 sock.send(data)
 print("send back to server: (%s)" % repr(data))
 break
data = sock.recv(1024)
print repr(data)
while 1:
 sock.send(data)
 print("send back to server: (%s)" % repr(data))
 break
sock.close()

make sure you run server and client in seperate files/terminals

• CKocar: I added do stuff here line this : req = struct.pack('12B', 0x00, 0x01, 0x00, 0x00, 0x00, 0x06, int(unitId), 0x03, 0xff, 0xc0, 0x00, 0x00) sock.send(req) I have this error """ sock.send(req) BrokenPipeError: [Errno 32] Broken pipe """

Answer 2

I think your question is "Why is the sock.connect() call hanging?". That's because by default it waits for a connection indefinitely. In other words, the call is 'blocking' by default. If you want to only wait up to 500 milliseconds for a connection, you need to specify this:

sock.connect(.5) #wait 500 milliseconds for a connection attempt

Also, see Python socket connection timeout