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by anonymous

RUT955 07.04

We are trying to read a modbus slave at address 250 and we get this error

 Request failed, result: Failed to set slave id for mb context

 Previous versions of the firmware supported this (Confirmed 07.02). This seems like validation issue.

We have modbus slaves that reply to such requests without issue from other modbus masters.

Is there a way to remove this validation and revert to previous behavior?

1 Answer

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by anonymous
Hello,

The maximum number of Modbus slave devices is specified in the Modbus standard itself. Modbus standard sets a limit on the maximum number of Modbus slave devices, which is 247. This means that Teltonika devices can support Modbus slaves up to 247, including 247.

It's possible that older firmware versions could have allowed for slave IDs just slightly over the limit of 247. However, improvements have been made to the Modbus functionality since then, and now, as a result, it conforms more closely to the Modbus standard.

Kind Regards,

Andzej
by anonymous
After reviewing the standard, the 247 limit you reference is for Modbus RTU, not Modbus TCP. In Modbus TCP the unit id is a single byte and the Modbus server can reply as it sees fit to 0-255 unit ids. In our case we have a vendor that replies to unit id 250 for a non-RTU device without conflicting with the RTU range on the other side of the bridge.

It seems like a fairly easy lift to revert this validation with no downside to allowing 0-255 in the slave_id field.
by anonymous
Hi,

Indeed, you are correct when it comes to Modbus TCP. I thought you were referring to Modbus RTU initially. Sorry about that.

I have looked at Modbus TCP and have observed the same behaviour. I will consult our RnD department to confirm whether this is intentional or not, and I'll keep you informed once I receive any updates.

Kind Regards,

Andzej
by anonymous
Hi,

As promised, I wanted to give you an update. The R&D team has provided me with information regarding the issue, and the plan is to make changes in the upcoming 7.5 firmware release. Specifically, the valid slave IDs will be expanded from the range of [1;247] to [1;255]. Unfortunately, we do not have a specific release date for this firmware at the moment.

I appreciate you informing us about this issue.

Kind Regards,

Andzej