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I have some questions about the external voltage measurement by FMB 120 and FMA 120.

FMB 120:

I measure the voltage with the <battery> command.

Only the power supply (VCC and GND pins) is connected to FMB 120. No voltage peaks in the car installation, power consumption during voltage measurement max 100 mA.

I get the following results:

[Voltage in SMS message] / [Voltage measured with a multimeter]:

12977 / 12.54 V

12727 / 12.50 V

11815 / 12.10 V (under the load heating system)

12282 / 12.50 V

690 / 00.00 V (disconnected VCC and GND)

Is it normal that the FMB 120 is so inaccurate? What is the point of setting the Trigger External Voltage in I / O if the measurements are not real?

Analog Input measurement error on 12V is 0.9%, additional error on 12V (range 2 0-30V) is 108mV, which does not give such discrepancies.

FMA 120

No <battery> command

How to measure the voltage in the car installation?

Do I have to connect + to AIN 1 with 150kΩ resistor? There is no other way?

How to read the value under AIN 1? With the <getio> command?

Regards

Adam

2 Answers

0 votes
answered by
Hello.

In short FMA device vs FMB - these are two completely different device families.

On FMA devices STM32F103 MCU is used, on FMB platform - SOC is used.

And because of this - on FMA devices since its general purpose MCU, we can dedicate its ADC for precise measurements. And thats what was done there.

On FMb on the other hand since its SOC - it is not designed to have precise AIN measurements and thus it has higher error margin. Nothing can be done here to improve AIN measurements @ FMB platfrom.

How to read the value under AIN 1? With the <getio> command?

https://wiki.teltonika.lt/view/FMB_getio

Basically command getio returns all IO information: all DIN's, all AIN's and all DOUT's.

Regards
0 votes
answered ago by
Hello Spiker,

Thank you for the question.

Q:Is it normal that the FMB 120 is so inaccurate? What is the point of setting the Trigger External Voltage in I / O if the measurements are not real?

A:If one uses external voltage as a trigger to indicate whether the car has ignition (which is what this parameter is meant for), the voltage jumps by 2V(12V vs 14V) and ~0.2V(claimed by your measurements) accuracy should be more than enough for the application.

On the otherhand AIN is usually used for fuel sensors and such, here the accuracy is important to know exact fuel level, therefore it is designed with higher accuracy in mind.

Q:How to measure the voltage in the car installation?

A:If external voltage parameter is enabled in the configuration, you can check it by using:

getparam 66

Please find additional information here: https://wiki.teltonika.lt/view/FMA120

If you have any further questions don't hesitate to ask. Alternatively, upvote the answer if it provided a solution to your question.

Have a great day. :)

Best regards,

Vytenis