RS485 Breakout with 48 V Passthrough Building Block Board — Part 5

RS485 Breakout with 48 V Passthrough Building Block Board — Part 5#

Today I carried out some more testing on the RS485 breakout boards, and things appear to be heading in the right direction.

The tests were still relatively simple at this stage, but useful. The aim was not to validate a complete communications protocol, but to prove that two boards could be configured as a basic RS485 transmitter and receiver pair, and that the transceiver section behaved as expected.

For the first test, I used two of my previous USB-C breakout boards with built-in 3.3 V regulators to power the RS485 boards. One RS485 board was configured as the transmitter, and the other as the receiver.

On both boards, I linked DE and /RE together using the jumper on JP7. This allowed each board to be placed clearly into either transmit or receive mode.

For the receiver board, I pulled the combined DE//RE control line low. This disabled the driver and enabled the receiver.

For the transmitter board, I pulled the combined DE//RE control line high. This enabled the driver and disabled the receiver.

This gave me a simple one-way test setup:

• one board driving the RS485 bus
• one board listening to the RS485 bus
• both boards powered independently from separate 3.3 V supplies
• the RS485 D0/A and D1/B lines connected between the two boards
• the RS485 ground/reference connected between the two boards

At this point, the goal was simply to confirm that a logic signal applied to the transmitter board would appear correctly at the receiver board output.

The LED indicators on the RO and DI lines gave me a quick visual indication that the signal was being passed across the link. Pulling DI high on the transmitter board caused the RO indicator on the receiver board to turn on. Pulling DI low caused the RO indicator on the receiver board to turn off.

So far, so good.

I also checked the voltages on the D0/A and D1/B pins. The readings were as follows:

These results appear to comfortably exceed the minimum differential input voltage needed for the receiver to detect a valid bus state, which is a good sign for this initial static test.

I then repeated the same test using approximately 20 m of unshielded Cat 6 cable between the two boards and obtained similar results, which was encouraging for a first cable-length check.

This is a useful first test because it strips the system back to the minimum number of moving parts. There is no microcontroller firmware, no serial protocol, and no software configuration involved. It is just the physical board, the SP3485 transceiver, the bus wiring, and some manually controlled logic levels.

That makes it much easier to check whether the hardware itself is behaving sensibly before adding more complexity.

This does not prove high-speed serial communication yet, but it does show that the basic bus states are being driven and received correctly.

The next step will be to move beyond static logic levels and start testing actual serial communication between two boards. Once that is working, I can start looking at longer cable runs, termination, biasing, and how the board behaves in a more realistic multi-node setup.

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