Connecting Two PCBs With a Bus: Do Not Pass When Flashing Yellow
During my school days, my favorite time of the day was when the bell rang and we were free to go home. While waiting for my school bus, I spent time messing around with my friends and would be so engrossed in playing that I’d occasionally miss my bus.
Naturally, my mom wasn’t too thrilled about having to pick me up after a busy day of work. Missing the school bus, fun as it may have initially seemed, resulted in some serious grounding. Without diving too deeply into the colorful scoldings I received in those days, let’s focus on an entirely different kind of bus: the bus you encounter in PCB design.
What is a “Bus” and Why Do You Need One in Your Design?
No man is an island and neither are your electronic designs. When you’re involved in designing advanced systems like an industrial controller or alarm controllers, your PCB needs to interface with external components like an LCD or a memory expansion board. In the strictest sense, a bus provides the necessary communication pathway for digital data in such cases.
Communications interfaces like Universal Serial Bus (USB), RS-232 and Controller Area Network (CAN) are digital buses commonly encountered in design. Besides these, there are memory buses that connect a microcontroller to various memory chips. Some buses carry data at very high speeds.
Along with these standard buses, you may encounter custom data buses during design. This can occur, for instance, when you have to connect two or more PCBs together to ensure they are all communicating with each other. This usually happens in controllers where the microcontroller is designed on a separate board or a design variation is achieved by swapping one of the PCBs.
For example, I once designed a customized MP3 player that needed to be separated into three different PCBs due to mechanical limitations. Similarly, medical gas panel designs also require two different PCBs for indication and sensing. In such cases, you would need to consider connecting the PCBs with a bus.
Practical Tips When Connecting PCBs With a Bus
Connecting two PCB involves more than just plugging them together with a random cable. Besides ensuring the interconnection is not susceptible to electromagnetic interference (EMI), you need to make the PCBs easy to assemble and install. Here are some helpful tips to minimize issues when connecting PCBs:
1. Choose the right connector (and cable)
When you’ve decided to separate your design into two PCBs, there are two key questions you need to ask:
What is the connector type?
Are the two PCBs connected with a cable?
Naturally, the shorter the data’s travel distance is, the better the signal integrity will be. In cases where the boards are linked with a cable, you’ll need to decide the type of cable to use. A common choice is the flat ribbon cable, especially if a huge number of individual data connections is needed. If your PCBs are meant to stack on each other, you’ll have the choose the right board-to-board connectors.
Ribbon cable is commonly used for interconnection between PCBs.
2. Minimize interference susceptibility
Regardless of the method that brings the PCBs together, you’ll want to decrease the chances of signal corruption due to EMI. This is especially true for data transmission via a ribbon cable. In actual installations, you would never know if the ribbon cable is placed to any electrically noisy equipment.
The best way to do this is to alternate the signal with ground connections on the ribbon cable. This helps to shield the signals from each other and from external interference. A good example is the twenty-pin Joint Test Action Group (JTAG) interface used for debugging microcontrollers.
3. Ensure sufficient power and ground connection
In some designs, one PCB supplies power to the other. A commonly overlooked aspect of custom bus design is providing sufficient power and ground connections. Ribbon cables and board-to-board connectors are limited in terms of the amount of current a single pin can carry. You’ll need to allocate sufficient pins for both positive voltage and ground. Failing to do so will result in overheating of the cable or connector and insufficient power being supplied to the second PCB.
4. Design for stability
This applies to PCBs that are attached using a board-to-board connector. While they may seem secure at your lab, the PCBs may loosen while being installed or experiencing vibration. To avoid these types of scenarios, it’s best to include screw holes in your design to fasten both PCBs with PCB stands. This will spare you from getting support calls just because the PCBs aren’t properly attached to each other.
5. Align accurately
The worst thing that could happen to a board-to-board design is to manufacture your PCBs and realize afterward that they don’t fit due to a slight misalignment in your design. To mitigate this possibility, you need to pay attention to details and use the right software tools.
Properly align board-to-board connectors on both PCBs.
Although there are several considerations to keep in mind, connecting your PCBs together with a bus doesn’t have to be complicated. Altium Designer provides intuitive alignment and measurement tools to help you eliminate common mistakes and account for potential design errors before your PCB is deployed. Manage your board layout more efficiently with effective PCB design software.
Still struggling with your inter-PCB connection? Talk with an expert at Altium today.