Unable to read the RS232 attached GPIO pins the same way as USB serial port on Pi3

Question

I maybe new to this, but I have the following configuration in place.

Step 1:

(Device 1){RS232}<=======(RS232 to USB cable)=======>{USB}(Pi)

• I have an RS232 to USB cable attached to the USB port of the Pi and a device which I have named Device 1 on the other end, and it's feeding data that looks like this(Reading in hex):

  F8 CD 0E 0F 00 11 00 00 1C 6D 01 01 8E A4 01 49 A4 91 02 00 44 38 7A 8B EC

• Now I disconnect the "RS232 to USB cable".

Step 2:

(Device 1){RS232}<=======(RS232 to GPIO)=======>{Pins: RX TX GND}(Pi)

• Similarly, I attach RS232 port of Device 1 to the GPIO RX TX pins of the Raspberry Pi 3. When I read the data from GPIO pins, it looks like the following(Reading in hex):

  A0 63 2F EF DD FF FF C7 25 FD FD E3 B7 FD 6F B7 DD FB FF 59 85 00

Results:

• The data received from the USB connection is correct, whereas the GPIO data is not. I expected both of the results to be the same, but they vary I don't know why?!

Here is my code:

#!/usr/bin/env python
import time
import serial

ser = serial.Serial(
   port='/dev/ttyAMA0',
   #port='/dev/ttyUSB0', #used for step 1
   baudrate=19200,
   parity='N',
   stopbits=1,
   bytesize=8,
   timeout=0.050,
   xonxoff=False,
   rtscts=False,
   dsrdtr=False
)
counter=0

while 1:
   x=ser.readline()
   if x:
       print x
       hex_list = ["{:02x}".format(ord(c)) for c in x]
       print hex_list

Could somebody please tell me what I should do to read the data from GPIO and get the same result as the USB data??

Answer 1

It seems you are missing the critical part of converting RS232 signal (which is not 3.3 V) to RPI GPIO compatible 3.3V Signal as described in this example: https://www.abelectronics.co.uk/kb/article/20/raspberry-pi-serial-port-usage http://raspberrypi.tomasgreno.cz/uart-to-rs-232.html

On GPIO header of RPi you can find a so called UART pins. In fact, it is normal >serial port you know from computers with only one change. UART uses 0 Volts for >logical 0 and 3,3 Volts for logical 1 while regular RS-232 uses -10V for 1 and >+10V for 0. This difference means that when you connect RS-232 device directly to >UART pins, RPi will surely not survive. For this reason, MAX3232CPE integrated >circuit was developed. It is a simple converter that converts voltage levels with >help of few capacitors. To build this, you will need:

You can either create your own converter like this people did: http://justanotherlanguage.org/content/jallib/tutorials/tutorial_serial_port_board

Many circuits will require some serial port communication, you may buy yourself a >rs232 to TTL adapter off the net for as little as $10, or you can build one >yourself. The max232 is a very popular chip. It converts your 5v circuit to the >12v required for serial communication to things like your PC. Many >microcontrollers have RX and TX output pins.

Or buy a converter designed for such purpose: https://cablematic.com/en/products/adapter-raspberry-pi-gpio-txrx-to-rs232-db9-male-MS050/

Specifications RS232 serial interface adapter card in a Raspberry Pi GPIO Tx/Rx system. Ideal for connecting serial devices in a Raspberry circuit: PLCs, industrial >controllers, etc. Transmission frequencyData ion: 230 Kbps. Input voltage: 2.7V to 5.25V. It has male DB9 connector, 4-pin connector, and connection cable.

Good luck

Answer 2

Apart from the voltage mismatch (RS232 is up to ±15V, although most devices which claim to be RS232 use much lower voltage unipolar signals), the polarities are different.

Even if you haven't already damaged the Pi it won't work.

There are RS232 <--> TTL devices, many of which have a 3.3V option.