python - Pyserial can't read device -

i'm trying read data off of sensor bought, using conversion module (ssi rs232). have module plugged windows laptop via usb/serial converter.

when use putty in serial mode, can send command $2rd , receive appropriate response sensor unit. when run script try same thing, unit returns: ''

here code using:

import sys import serial import time  ser = serial.serial(     port='com4',      baudrate=9600,      timeout=1,     parity=serial.parity_none,     stopbits=serial.stopbits_one,     bytesize=serial.eightbits, )    while true:     ser.write('$2rd'.encode())     #time.sleep(1)     s = ser.read(26)     print s 

a few other notes:

• i've tried variations using flushinput, flushoutput, sleeping, waiting, etc...nothing seems help.
• i know have com ports right/the hardware works in putty, pretty sure code.
• i've tries 13,400 baud no difference in outcome.
• if connect tx , rx lines usb, can read command i'm sending...so should @ least getting rs232/ssi conversion device.

s = ser.read(26) should ser.read(size=26) since takes keyword argument , not positional argument.

also, can try set timeout see sent after specific time because otherwise function can block if 26 bytes aren't sent specified in read docs of pyserial :

read size bytes serial port. if timeout set may return less characters requested. no timeout block until requested number of bytes read.