Imported from GitHub: bvarner/pi485 · commit 56a1a36 · license MIT

Description

An open-source TTL UART <-> RS485 converter, intended for a Raspberry Pi or Arduino

README

Pi485 Standalone Module

This is a stand-alone module for RS-485 communication, utilizing a MAX485 IC and a NE555 Timer IC. The PCB for this project is the most 'simple' of the two to etch yourself. Most of the pads are oversized, the copper traces are significantly wider and spaced far enough that it's possible to create a PCB with a resist marker by hand.

PCB Layout

Assembled

Schematic

Design Goals

• Simple / Easy To Understand Circuit
• Easily fabricated at home with simple tooling
• Reasonably inexpensive to source parts for and build (~$20 or less)
• Latch the DE / RE pins on the Max485 based upon the UART TX activity, but allow that to be disabled (force read-only mode)
• Include jumper termination for the A/B lines of the RS485 bus.

To achieve these goals the PCB layout in this repository is a single-side clad board, components on the front, copper on the back. The traces are large (0.4mm!), the pads are oversized where possible, and I've allowed plenty of clearance between traces and pads, making it possible to touch-up a hot-iron transfer with an ink-resist pen. You may even be able to draw the existing tracks with a pen free-hand.

For tooling, you'll need the 'standard' PCB etching supplies; a bottle of Ferric Chloride, some isopropyl alcohol, gloves, a dish to do the etching, and an appropriately sized drill bit. (I had a 0.040" bit in a set) I'd recommend you have a drill press, too.

I highly recommend doing a hot-iron transfer of laser printer toner. Print the SVGs using a laser printer onto glossy photo (or magazine) paper. Heat the board with an iron, then stick the printout to the board. Press the printout firmly to the board using the hot iron (you're goal is to melt the toner to the board surface), and then start soaking with water and continue steaming. Firm pressure and heat will do a good job of sticking the printout onto the board. It works surprisingly well. Once you've steamed the paper (or soaked it in water) and it starts to fall of the board, you can touchup any trace or pad issues with a resist pen, and etch the board.

I also use a kitchen oven to bake the board once it's tinned. Getting it up to ~425 degrees F does a nice job of reflowing the solder, which makes it much easier to drill the holes.

Circuit Analysis / Explanation

TTL Serial Communication Basics

TTL signaling of serial data idles 'high'. When data is being transmitted on the TX pin, it first drops 'low' (a start bit), transmits it's data, and then sends a stop bit. In TTL, a logical 0 is GND. A Logical 1 is +V. But when there are no transmissions taking place, the pin holds 'high', at +V. I personally found this SparkFun tutorial to be highly educational on the nuances of TTL. If your protocol doesn't include a start / stop bit, this can be problematic for converting TTL transmissions to RS-485.

Schematic Breakdown

The current schematic is fairly straightforward.

All of the resistor values connected directly to LEDs should be double-checked for your LED selections.

Power Input / Signal Connections

Starting in the upper left quadrant of the schematic, you have the input header pins (P1). There's a second pair of headers for an additional +5vdc and ground connections (P2). I use the second two-pin header to connect my power supply, then connect the other pins to the Pi providing power to the RaspberryPi and connecting the UART pins and 485PWR signal line.

RX / TX LED Signaling

The RX & TX lines are tapped with 270ohm resistors to pull the 5vdc down to an appropriate level for tripping the NPN transistors in the lower left side of the schematic. When the voltage on the RX / TX pins goes LOW (signaling data, not idle), the NPN halts the flow of electricity from the Collector to the Emitter (opens the switch), leaving the only path to ground being through the LEDs. Make sure you size your R3 and R7 resistors appropriately for your LEDs. The NPN transistors in this circuit are specced as 2N3904's, but 2N4401s will work just as well.

Power Filtering & Power LED

Above the RX / TX LED section, there's a small block with two transistors (an NPN and a PNP) working together to create a 'high-side' power switch for the entire circuit. The PNP transistor switches the +5v supply for the rest of the circuit, based on the on/off state of the NPN. Using the NPN to control the PNP allows us to drive the 5v line with a 3.3v GPIO, and to do it in a '1' is on, '0' is off, semantic.

If you'd rather the circuit always be 'on', omit these components, or connect pin 5 on the P1 header to the 5V or a 3.3V line.

Looking to the lower-right portion of the schematic, you'll find several decoupling filter capacitors and a power LED.

UART TX Connections

The TX line is connected to the trigger pin of a 555 timer, and then to the Data Input pin of the Max485.

The 555 timer circuit is a slightly modified missing pulse detector, which you'll find right smack in the middle of the schematic.

When the TX power drops below the threshold defined by the 3.9k resistor (R2), the output of the 555 goes HIGH. After a very short delay, (it switches ~38400hz) the output will go back low, unless the the TX line input is still low. In a typical missing pulse detector, the 555 output will go back low regardless of the TX line (555 trigger) input. The addition of the shottky diode changes this configuration, and holds the output steady so long as the input is steady. The diode has to be capable of switching reverse voltage bias fast enough to keep the input lower than the threshold / discharge output, but without affecting the input signal line. Hence the use of the BAT43, rather than a 'normal' silicon diode.

The output of the 555 timer is used to set the input on the DE / RE (Driver Enable / Receiver Enable) pins on the max485. When HIGH, the Driver Enable is set, putting the Max485 into 'master' mode and broadcasting on the 485 bus. When LOW, the RE pin (which has inverted logic on a Max485) is enabled, putting the Max485 into 'receive' mode.

If there is a jumper present on the CTRL pin (5) of the 555 timer, the 555 output will always be 'LOW' effectively holding the Max485 in Receive mode forever.

UART RX Connection

Moving along the RX line, it's connected to the Data Output pin of the Max485.

Max485 Output

The output pins of the Max485 are configured to setup a high-resistance reference against the current 'ground', and to allow the termination (120 Ohm) resistor to be removed from the circuit.

Putting it all together...

The circuit provides basic decoupling / power filtering, some passive LED monitoring of RX / TX lines, and a latch-based DE / RE driver logic for the Max485, along with a basic resistance network to setup the A/B differentials on the RS-485 bus.

Analysis of other products / options

I am not a professional EE. In reality I'm a software engineer who got into this out of curiosity. I have always detested black-boxes (things which I cannot take apart or understand completely) and it's my nature to dismantle, analyze, evaluate, and eventually take that knowlege and apply it to things I synthesize. I consider electronics a 'hobby'. My comments here are my own personal opinions and are not an endorsement or meant to dissuade or persuade your purchasing decisions.

Build Instructions

1. Start off by customizing the resistor values for your LEDs. I used [http://ledcalc.com/] to help inspire my choices.
2. Use KiCad's PCB editor (pcbnew) to customize any footprint changes.
3. Print the back copper layer on a laser printer, using glossy paper.
4. Print the front layers on a laser printer, using glossy paper.
5. Cut your board to size (63mm x 46mm)
6. Transfer the copper printout using a hot iron and some water.... (youtube reference coming). Touchup with a resist pen if necessary.
7. Etch the board.
8. Clean the board.
9. Tin the board.
10. Reflow the board (bake it in an oven).
11. Drill the component holes.
12. Place & solder components.
13. Clean up, and enjoy!