RS485 transmit errors

[sebastieng]

Hello,

I am trying to setup a RS485 gateway on a raspberry with an arduino node.

Raspberry gateway is build with

./configure  --my-gateway=ethernet --my-port=8011 --my-transport=rs485 --my-rs485-serial-port=/dev/ttyAMA0 --my-rs485-de-pin=37

The arduino sketch is configured with

#define MY_DEBUG
#define MY_RS485

#define MY_RS485_DE_PIN 2

#define MY_RS485_BAUD_RATE 9600
#define MY_BAUD_RATE 9600
#define MY_NODE_ID 2

On each RS485 I have a 120 ohm resistor. The arduino and the raspberry are very close so the wires are short

The parent request from the node to the gateway fails. I have added extra logging on each side, and I see following issue

• the parent request is received by the gateway
• but the response to the node is corrupted.

The gateway write following bytes

• 1 -> SOH
• 2 -> destination node
• 0 -> source address
• 88 -> command
• 8 -> message length
• 2 -> STX

The node receives

• 1 -> SOH
• 65534-> destination node -> ERROR
• 0 -> source address
• 65496-> command-> ERROR
• 8 -> message length
• 2 -> STX

As you can see some bits are corrupted
Expected destination node =2 (in binary 0000 0000 0000 0010), while we received 65534 (in binary 1111 1111 1111 1110). The leading bits are 1 instead of 0
Same issue about command byte.
It is 100% reproductible, this message is always received with same values so I don't think it is caused by noise or something like that.

Any idea about the root cause?

[fsgraz]

@sebastieng too much load ? one 120ohm on the far one should be enough

[sebastieng]

FYI, I have found my error

The description of my setup was not totally correct, I don't use an arduino but a atmega328p standalone.
The root cause was the use of the internal oscillator, it is not enough accurate. I am now using an external quarz and it is working fine

[OldSurferDude]

@sebastieng Thanks for letting us and future readers know.

-OSD