gps serial read error

I receive the folowing error while I read gps information from my MyPal
636N:

('SerialException', '<no args>', ['  File "\\SD
card\\work\\python\\serial-teszt.py", line 32, in <module>\n   
rv=s.read(50)\n', '  File "\\SD Card\\Program
Files\\Python25\\ceserial.py", line 205, in read\n    return
self.__buffered_read(size)\n', '  File "\\SD Card\\Program
Files\\Python25\\ceserial.py", line 248, in __buffered_read\n    raise
SerialException, windll.coredll.GetLastError()\n'])

Source code:

# -*- coding:iso8859-2 -*-
import sys
import ctypes
import winbase
import ceserial
import exch
latin2_decoder = lambda s: str(s) #unicode(s, "iso8859-2", "replace")

def gprmc_test(row_ary):
    return (row_ary[0] == "$GPRMC") and (row_ary[1]!="") and
(row_ary[3]!="") and (row_ary[5]!="")

def get_pos(row_ary):
    return
{'rtime':float(row_ary[1]),'lon':float(row_ary[3]),'lat':float(row_ary[5])}

def rec_pos(data):
    cur.execute("insert into coords_rec (rectime,long,lat) values
(%f,%f,%f)"%(data['rtime'],data['lon'],data['lat']))

from sqlite3 import dbapi2 as sqlite
con=sqlite.connect("\\SD Card\\work\\cdata.db",isolation_level=None)
con.text_factory=latin2_decoder
cur=con.cursor()

s=ceserial.Serial(port="COM5:",baudrate=4800)
s.open()
of=open('\\SD Card\\work\\python\\tmp.txt',"w")
rv=s.read(50)
cnt=0
c2=0
while (len(rv)>0) and (cnt<10000) and (c2<5):
  try:
    of.write(rv)
    gps_ary=rv.split(',')
    if (gprmc_test(gps_ary)):
      rec_pos(get_pos(gps_ary))
      print "(%f,%f,%f)"%(rtime,lon,lat)
      #break
    cnt+=1
    rv=s.read(50)
  except Exception, e:
    c2=c2+1
    print "%d. exception: "%c2
    print exch.formatExceptionInfo()

con.close()
of.close()
s.close()