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Hi everybody,

since I was very interested when I found the opentherm Gateway I bougth myself the RS232 kit.
I solder everything and checked the working by the troubleshooting guide.

At the X1 therminal the current was a bit high : 8 mA and 28 mA.

The terminal is not able to communicating to OTmonitor or putty. connected to a USB2Serial or normal COM-port.
When Thermostat is connected it will power up and tell me "wrong connected".

The kit is delivered with 4x BC-557B instead of BC-558A

Can anyone tell me what's the difference and if this can be the problem.

One more question is, for making the USB version can I just use a regular USB cable or is a special TTL USB cable needed?

thanks in advance.

Regards,
Herwin

Using the RS232 version of the OTGW with a standard USB serial cable is a perfectly reasonable setup. Using the special TTL USB cable, you can save some money because you don't need IC2 and C3 - C6, and the cable may be a bit cheaper too. On the flip side, people using the TTL cable have reported issues when the cable is not powered from the USB side. You shouldn't have those problems with a normal USB serial cable. So, as with most things, YMMV.

You can download the data sheets for the BC557B and BC558A and look for the differences just as well as I can. IIRC there's a difference in maximum voltage the device can handle (which should be high enough on both), the frequency (also plenty for this low frequency application) and the amplification. This last one may give some deviations, but the trouble shooting page provides some tips on how to compensate for that.

But let's start at the beginning. The serial communication doesn't depend on the transistors. So if that's not working, the problem is elsewhere. Please follow the serial interface part of the troubleshooting instructions to fix that first. Once that's up and running it's much easier to determine where the OpenTherm communication is going wrong.

hvxl wrote:Using the RS232 version of the OTGW with a standard USB serial cable is a perfectly reasonable setup. Using the special TTL USB cable, you can save some money because you don't need IC2 and C3 - C6, and the cable may be a bit cheaper too. On the flip side, people using the TTL cable have reported issues when the cable is not powered from the USB side. You shouldn't have those problems with a normal USB serial cable. So, as with most things, YMMV.

You can download the data sheets for the BC557B and BC558A and look for the differences just as well as I can. IIRC there's a difference in maximum voltage the device can handle (which should be high enough on both), the frequency (also plenty for this low frequency application) and the amplification. This last one may give some deviations, but the trouble shooting page provides some tips on how to compensate for that.

But let's start at the beginning. The serial communication doesn't depend on the transistors. So if that's not working, the problem is elsewhere. Please follow the serial interface part of the troubleshooting instructions to fix that first. Once that's up and running it's much easier to determine where the OpenTherm communication is going wrong.

Hi thanks a lot for your answers.

I have done some testing using the troubleshooting guide. Here are the result :
POWER SUPPLY
All IC’s removed no thermostat or boiler connected no power.
X3=3,05kΩ
Main power supply connected
IC1 pin5 to pin14 = 4,95v
IC2 pin15 to pin 16=4,95v (5v
OPENTHERM INTERFACE
X1=24,8v (18-34,5V)
125Ω resistor connected to X1
X1 U=0,79V = 6 mA (5mA-9mA)
IC1 pin3 shorted to pin5
X1 U=2,55V = 20,4 mA (17mA-23mA)
OK1 placed, X1 connected to X2 using 2 wires
X2=16,55V
IC1 pin 2 shorted to pin 14
X2=5,39V
IC1 pin 18 to pin 5 = 0V (<1.25V)
IC1 pin 3 shorted to pin 5
IC1 pin 18 to pin 5 =1,65V (>1.25V)
SERIAL INTERFACE
IC2 Placed
IC2 pin 2 = 8,74V
IC2 pin 6 = -8,33V
Serial pin 2 to pin 5 = -8,33V

It looks like voltage pin 6 is a little bit to low could this be the problem?
Can it be a broken MAX232?

regards,
Herwin

Hi all,

I figured out what was wrong.
serial flat cable delivered in the package was connected wrong. soldered RS232 Cable directly to to PCB : working.
thank you for your help !

Regards,
Herwin