we create custom module to enable pos to work offline by checking connection every one minute 

but after first three orders, pos can not send orders to database , and the led show red color 

this snapshot from my code to check internet connection to server

function hostReachable() {

  // Handle IE and more capable browsers
  var xhr = new ( window.ActiveXObject || XMLHttpRequest )( "Microsoft.XMLHTTP" );
  var status;
  var server = window.location.hostname;
  if (window.location.port != '') {
    server += ':'+window.location.port;
  }

  // Open new request as a HEAD to the root hostname with a random param to bust the cache
  xhr.open( "HEAD", "//" + server + "/?rand=" + Math.floor((1 + Math.random()) * 0x10000),false );

  // Open new request as a HEAD to the root hostname with a random param to bust the cache
//  xhr.open( "HEAD", "//" + window.location.host + "/?rand=" + Math.floor((1 + Math.random()) * 0x10000), true );
  // Issue request and handle response
  try {
    xhr.send();

    if (xhr.status >= 200 && xhr.status < 304) {
//        alert("connected");
        return true;
    } else {
            return false;
    }
  } catch (error) {
    return false;
  }

}

any help please ?

edit:snapshot from log file

2014-10-02 11:31:12,628 2089 ERROR Tor-9-24 openerp.sql_db: bad query: insert into "account_bank_statement" (id,"user_id","name","company_id","journal_id","state","period_id","date",create_uid,create_date,write_uid,write_date) values (290,1,'/','1',NULL,'draft',11,'2014-10-02 11:31:12',1,(now() at time zone 'UTC'),1,(now() at time zone 'UTC'))
Traceback (most recent call last):
  File "/home/belalsalah/ERP/server/openerp/sql_db.py", line 226, in execute
    res = self._obj.execute(query, params)
IntegrityError: null value in column "journal_id" violates not-null constraint

2014-10-02 11:31:12,630 2089 ERROR Tor-9-24 openerp.netsvc: Integrity Error
The operation cannot be completed, probably due to the following:
- deletion: you may be trying to delete a record while other records still reference it
- creation/update: a mandatory field is not correctly set

[object with reference: journal_id - journal.id]
Traceback (most recent call last):
  File "/home/belalsalah/ERP/server/openerp/netsvc.py", line 296, in dispatch_rpc
    result = ExportService.getService(service_name).dispatch(method, params)
  File "/home/belalsalah/ERP/server/openerp/service/web_services.py", line 626, in dispatch
    res = fn(db, uid, *params)
  File "/home/belalsalah/ERP/server/openerp/osv/osv.py", line 190, in execute_kw
    return self.execute(db, uid, obj, method, *args, **kw or {})
  File "/home/belalsalah/ERP/server/openerp/osv/osv.py", line 174, in wrapper
    netsvc.abort_response(1, _('Integrity Error'), 'warning', msg)
  File "/home/belalsalah/ERP/server/openerp/netsvc.py", line 71, in abort_response
    raise openerp.osv.osv.except_osv(description, details)
except_osv: ('Integrity Error', 'The operation cannot be completed, probably due to the following:\n- deletion: you may be trying to delete a record while other records still reference it\n- creation/update: a mandatory field is not correctly set\n\n[object with reference: journal_id - journal.id]')
> /home/belalsalah/ERP/server/openerp/netsvc.py(71)abort_response()
-> raise openerp.osv.osv.except_osv(description, details)
(Pdb) 2014-10-02 11:31:20,068 2089 INFO Tor-9-24 werkzeug: 127.0.0.1 - - [02/Oct/2014 11:31:20] "POST /web/dataset/call_kw HTTP/1.1" 200 -
2014-10-02 11:31:21,444 2089 ERROR Tor-9-24 openerp.addons.pos_multi_currency.pos_multi_currency: Dataaaa ############### : {'journal': 118, 'payment_date': '2014-10-02 10:59:09', 'amount': 10.47, 'payment_name': False, 'statement_id': 284}
2014-10-02 11:31:21,775 2089 ERROR Tor-9-24 openerp.sql_db: bad query: insert into "account_bank_statement" (id,"user_id","name","company_id","journal_id","state","period_id","date",create_uid,create_date,write_uid,write_date) values (291,1,'/','1',NULL,'draft',11,'2014-10-02 11:31:21',1,(now() at time zone 'UTC'),1,(now() at time zone 'UTC'))
Traceback (most recent call last):
  File "/home/belalsalah/ERP/server/openerp/sql_db.py", line 226, in execute
    res = self._obj.execute(query, params)
IntegrityError: null value in column "journal_id" violates not-null constraint

2014-10-02 11:31:21,776 2089 ERROR Tor-9-24 openerp.netsvc: Integrity Error
The operation cannot be completed, probably due to the following:
- deletion: you may be trying to delete a record while other records still reference it
- creation/update: a mandatory field is not correctly set

[object with reference: journal_id - journal.id]
Traceback (most recent call last):
  File "/home/belalsalah/ERP/server/openerp/netsvc.py", line 296, in dispatch_rpc
    result = ExportService.getService(service_name).dispatch(method, params)
  File "/home/belalsalah/ERP/server/openerp/service/web_services.py", line 626, in dispatch
    res = fn(db, uid, *params)
  File "/home/belalsalah/ERP/server/openerp/osv/osv.py", line 190, in execute_kw
    return self.execute(db, uid, obj, method, *args, **kw or {})
  File "/home/belalsalah/ERP/server/openerp/osv/osv.py", line 174, in wrapper
    netsvc.abort_response(1, _('Integrity Error'), 'warning', msg)
  File "/home/belalsalah/ERP/server/openerp/netsvc.py", line 71, in abort_response
    raise openerp.osv.osv.except_osv(description, details)
except_osv: ('Integrity Error', 'The operation cannot be completed, probably due to the following:\n- deletion: you may be trying to delete a record while other records still reference it\n- creation/update: a mandatory field is not correctly set\n\n[object with reference: journal_id - journal.id]')
> /home/belalsalah/ERP/server/openerp/netsvc.py(71)abort_response()
-> raise openerp.osv.osv.except_osv(description, details)
(Pdb) 2014-10-02 11:31:29,153 2089 INFO Tor-9-24 werkzeug: 127.0.0.1 - - [02/Oct/2014 11:31:29] "POST /web/dataset/call_kw HTTP/1.1" 200 -
2014-10-02 11:31:30,346 2089 ERROR Tor-9-24 openerp.addons.pos_multi_currency.pos_multi_currency: Dataaaa ############### : {'journal': 118, 'payment_date': '2014-10-02 10:59:09', 'amount': 10.47, 'payment_name': False, 'statement_id': 284}
2014-10-02 11:31:30,685 2089 ERROR Tor-9-24 openerp.sql_db: bad query: insert into "account_bank_statement" (id,"user_id","name","company_id","journal_id","state","period_id","date",create_uid,create_date,write_uid,write_date) values (292,1,'/','1',NULL,'draft',11,'2014-10-02 11:31:30',1,(now() at time zone 'UTC'),1,(now() at time zone 'UTC'))
Traceback (most recent call last):
  File "/home/belalsalah/ERP/server/openerp/sql_db.py", line 226, in execute
    res = self._obj.execute(query, params)
IntegrityError: null value in column "journal_id" violates not-null constraint

2014-10-02 11:31:30,686 2089 ERROR Tor-9-24 openerp.netsvc: Integrity Error
The operation cannot be completed, probably due to the following:
- deletion: you may be trying to delete a record while other records still reference it
- creation/update: a mandatory field is not correctly set

[object with reference: journal_id - journal.id]
Traceback (most recent call last):
  File "/home/belalsalah/ERP/server/openerp/netsvc.py", line 296, in dispatch_rpc
    result = ExportService.getService(service_name).dispatch(method, params)
  File "/home/belalsalah/ERP/server/openerp/service/web_services.py", line 626, in dispatch
    res = fn(db, uid, *params)
  File "/home/belalsalah/ERP/server/openerp/osv/osv.py", line 190, in execute_kw
    return self.execute(db, uid, obj, method, *args, **kw or {})
  File "/home/belalsalah/ERP/server/openerp/osv/osv.py", line 174, in wrapper
    netsvc.abort_response(1, _('Integrity Error'), 'warning', msg)
  File "/home/belalsalah/ERP/server/openerp/netsvc.py", line 71, in abort_response
    raise openerp.osv.osv.except_osv(description, details)
except_osv: ('Integrity Error', 'The operation cannot be completed, probably due to the following:\n- deletion: you may be trying to delete a record while other records still reference it\n- creation/update: a mandatory field is not correctly set\n\n[object with reference: journal_id - journal.id]')
> /home/belalsalah/ERP/server/openerp/netsvc.py(71)abort_response()
-> raise openerp.osv.osv.except_osv(description, details)
(Pdb) 2014-10-02 11:31:50,372 2089 INFO ? werkzeug: 127.0.0.1 - - [02/Oct/2014 11:31:50] "HEAD /?rand=114483 HTTP/1.1" 200 -
2014-10-02 11:31:50,389 2089 INFO Tor-9-24 werkzeug: 127.0.0.1 - - [02/Oct/2014 11:31:50] "POST /web/dataset/call_kw HTTP/1.1" 200 -
2014-10-02 11:32:20,368 2089 INFO ? werkzeug: 127.0.0.1 - - [02/Oct/2014 11:32:20] "HEAD /?rand=108520 HTTP/1.1" 200 -
2014-10-02 11:32:20,385 2089 INFO Tor-9-24 werkzeug: 127.0.0.1 - - [02/Oct/2014 11:32:20] "POST /web/dataset/call_kw HTTP/1.1" 200 -