I'm an electrical engineering students and for a school project I need to understand a circuit schematic. First, I will provide some context. The school project I'm working on is the modification of a sniffer in order to add the decoding of another protocol. The one already implemented is a proprietary serial protocol very much like the Dali protocol. The Dali protocol is used for Lighting. In my case a control device send frames to a security lighting. The sniffer is plugged in between. Frames looks like that:
The Idle state is high and it uses biphased code. First I want to investigate the hardware to see how the first one was implemented and what kind of modification will be needed but I'm pretty overwhelmed by the amount of complexity of the schematic so I will try to understand it one small part at a time. Here is a part of the schematic:
The full schematic is 4 pages long so I don't know if it would not be too much. The circuit was designed by a deutsche engineering team.The High Level voltage is 24 V and the low level is <= 4.5 V .The DALI_line_1 is the input which transmit the frames.DALI_line_OUT_1 is the output that will send the frame to the security lighting. The DALI_RX_1 is connected to the pin of a microcontroller that will read logical level to decode the frames and print it on a terminal.
I'm currently interested in understanding the part commanded by the optocoupler V50. This is I assume the part that produce the logical level. And this is what I'm currently trying to understand. I have basic electronics knowledge so I know what every component does, but I don't understand why they are assembled the way they are.
- I don't understand what the capacitors C65 and C64 are used for since they will block the current.
- What is the middle branch with R91 and C64 supposed to do?
- When the optocoupler is on, will the current flow through the base of the transistor V53 or to the ground through R89? For me the BE junction doesn't have any resistance, so the current will keep going through the base.
I'm pretty lost so any help is welcome. Thanks in advance.