SIM800C based modem issue

Thread Starter

nikoloz qvrivishvili

Joined Jan 29, 2021
Dear community members,

In my hands there is a 'PLC-alike' device originated from some local very little-known manufacturer.
Purpose of this device is transmission of binary and telemetry data from remote sites(HV substations etc) through GPRS modem with use of IEC 60870-104 protocol to some SCADA-alike software.
There are more than 400 devices of this type being used for a while(4 years). Most of them appx. 2/3 do operate properly, the rest after some years(1-2) of proper operating lost GPRS connection with the server(do not respond to ping (<Sim IP>) command). Please have a look at the device pics below:
sim800c inside.jpg
The "IMX287" consists of two PCB's. The first pcb above is modem itself with power linear rectifier MIC29302 which feeds SIM800C(and the rest). This PCB has a slot for a SIM-card. There are circuits for the binary inputs on this PCB, they work properly. The second PCB is a computer. The soft runs from SD card.
The problem is after being powered on, it fails to establish connection. Software keeps modem attempting to establish connection, when it fails it waits for sometime, and then restarts modem, etc. Please have a look into the 'schematic diagrams' archive. The file 'Схема платы' refers to modem pcb alongside with MIC29302 power rectifier.
I have been adviced how to diagnose faults of this device by the individuals who have designed modem PCB, assembled and programmed it without much success. And therefore they suggested me to have a check the voltage value before U1(MIC29302) U1(5V) and after U1 'Vbat' that should be within 3.4-4.4 V range. Also within 1 min after powered on there must be a short negative slope on the PWRKEY path. Please have a look into archive 'Oscilloscope samples' regarding U1, Vbat and PWRKEY signals.
Dear gentlemen, my objective is to diagnose the possible reasons of the faulty operation. And performing possible repair if the reason is discovered.
I believe with the precious advices of yours, at least possible reasons of faults could be found.

Yours faithfully.


Thread Starter

nikoloz qvrivishvili

Joined Jan 29, 2021
Good afternoon, dear community members!
In this post I try to tell you about my research I undertook to diagnose SIM800C possible issues.
I hope this may be useful to individuals who is exposed to operating modems.
In order to be sure modem is operational when:

1. Vbat is within 3.4 - 4.4 V range.
2. The PWR_KEY should generate pulse as on figure below.
3. There is the exchange with the sim-card(modem and sim-card).
4. The SIM card is powered with SIM_VDD voltage. SIM_VDD could be 1.8, 3.3 or 5 V.
5. Each time SIM_VDD is applied, modem injects pulses into SIM_CLK line.
6. SIM_CLK is replied with SIM_DATA.
7. After all there is SIM_DET line.
Its potential is 0 when SIM is inserted, if SIM is off its potential is close to Vbat value.
SIM_DET is a sign of the presence or absence of contact with the SIM card in the slot.

Depending on the measurment results:
1. Vbat is out of 3.4-4.4 range - we deal with it.
2. PWR_KEY is absent - software issue, contact, controller, link issue.
3. Vbat and PWR_KEY is OK, but SIM_VDD and/or SIM_CLK is absent - modem is perished.
4. SIM_VDD and SIM_CLK is OK, but SIM_DATA is absent then SIM card is perished or contact lost.

After having reflected on abovementioned, I glanced at the SIM800C pinout:

1. The first signal I targeted was SIM_DET. It showed when it's embedded it values 0 and 2.831 V extracted.
2. The second was sweety pair Vbat(red beam) and PWR_KEY(yellow):
The Vbat and PWR_KEY is OK.
3. The next pair is SIM_VDD(red beam) versus SIM_CLK(yellow):
It is cleared that SIM_VDD is acompanied by SIM_CLK serie as expected. The modem is alive and has reference to SIM card.

4. STATUS(red beam) versus SIM_DATA(yellow)
It shows as STATUS changes SIM_DATA generates serie of pulses carrying data to exchange.

All values above seems very happy and properly acting. The modem itself seems to be operational.

But still the command "ping -t" is not echoed by the recipent(modem) as supposed to be.

Maybe this issue is not modem issue at all, or there should be some extra checking taken, you can kindly direct me to do?

Yours faithfully.