HOWTO: Building A Stripboard Comms Controller

Introduction

The Comms controller is the main interface between the host PC and the rest of the RepStrap or RepRap. See RepStrap for an overview of all the components.

Requirements

Tools and consumables

• A soldering iron and accessories (see Electronics Fabrication Guide).
• Solder
• Electric drill or at least a suitable drill bit
• Some wire for jumpers/links
• Wire strippers
• Small pliers or tweezers

Components

• C1: 100μF eletrolytic capacitor (with a voltage rating of 30V or higher).
• C2-C5: 1μF eletrolytic capacitors (with a voltage rating of 30V or higher).
• Z1: MAX-232 or equivalent chip
• Z2: 7805 voltage regulator
• Connectors. A simple low cost choice is to use header pins and matching sockets. Try to get keyed pins and sockets that prevent you from putting the connector on backwards, because if you do it will most probably destroy the module and possibly other connected modules. Take great care! Alternatively, you can directly solder wires into the board after putting the modules on the machine in which case you don't need any connectors. If you use connectors, you need:
  • J1: two pin connector (used as a power connector)
  • J4: three pin connector (used to connect to the PC serial port)
  • J2, J3: four pin connector (used to connect to the network of devices)
• A piece of stripboard with 2.54mm (0.1") spaced continuous track. It should be a minimum of 21 holes along the track direction and 12 holes sideways. It is a good idea to provide some extra space to make screw holes for mounting.

Schematic

This is the schematic diagram of the circuit you will be building. See reading a schematic^? to understand the various parts.

Preparing the board

The strips on the back of the board need to be "separated" at certain points. The easiest way to do this is to use a small electric drill with a drill bit around 3-4mm in diameter. Don't drill all the way, just far enough to cut the copper tracks. See the Electronics Fabrication Guide for more details.

This image shows the rear of the stripboard so you can see the points that were separated.

The points marked in black are where you should separate the tracks.

Note: This is the minimum size. It is recommended that you leave sufficient additional space on all sides to allow for easy mounting. Around 20mm will be ample.

Important: There are some voltages that can and will damage components if you aren't careful. Double and triple check that:

• You have completely separated the necessary tracks. Use a multimeter or continuity checker if you have one.
• You have not connected any two adjacent tracks with shards of copper. It is a good idea to scrape down the channels to ensure there are no fragments. Again, check this with a multimeter or continuity checker.

Making the board

Solder the components into the board as pictured below. See the Electronics Fabrication Guide if you're new to soldering or component placement.

Pay particular attention to:

• Z1 (the MAX-232) chip orientation. The notch should be at the top, as pictured.
• Z2, the 7805 voltage regulator. The metal tab should be to the left as pictured.
• The capacitor polarity. Pay attention to the + and - indicators and match the capacitor. There is usually a band down one side of the capacitor that has minus (-) symbols written down it.

The 12V line must carry quite high current, so you should use fairly heavy duty wire for the link between holes B17 and E17.

Board connections

The connector pins are numbered from the top of the board down, so for example the pin in hole D8 is pin number 1 of connect J4.

• J1: Power connector
  1. Ground
  2. +12V
• J2: Incoming comms
  1. Comms channel
  2. +12V
  3. +5V
  4. Ground
• J3: Outgoing comms
  1. Comms channel
  2. +12V
  3. +5V
  4. Ground
• J4: PC Serial connector
  1. Ground
  2. To receive on PC (RX)
  3. To transmit on PC (TX)

Variations

If you're planning on using a PC power supply to run your RepRap, then you can use the 5V supply that it provides instead of using a 7805. This is a simple change and is left as an exercise to the reader

Even if you do use a PC power supply, you can just connect the 12V line and use the module as depicted.

Troubleshooting

Nothing here yet

If you experience trouble, please share the problems (and resolutions) with us.

-- SimonMcAuliffe - 02 Feb 2006
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