Serial adapter for 3.3V TTL

Introduction

This document describes how to build a converter from 3.3V TTL levels to RS232, which allows interfacing to a standard PC 9-pin D-sub port.

Bill of materials (BOM)

To build the serial adapter you will need the following parts:

  • 1 x piece of solderable breadboard, predrilled with standard 0.1" spacing
  • 1 x MAX3232 dual driver/receiver
  • 1 x 16 pin DIL socket (optional)
  • 5 x 0.1uF/10V polarized capacitors
  • 1 x 9 pin D-SUB male connector
  • up to four connectors for the serial header in your NAS box
  • some wire

In this description I am using a board where stripes of three holes are connected with copper. To connect to the NAS serial port I have glued together two header plugs with three holes each, forming a connector for a 2x3 serial header, suitable for Synology and QNAP. Depending on the kind of serial header in your NAS box you will have to build a suitable plug.

Schematic

This is the converter circuit we have to build, drawn using gschem from the gEDA suite.

Converter schematics
  • J1 is the plug for the serial header. Pin 1 is 3.3V, pin 2 is GND, pin 4 is TX. and pin 6 is RX. Adapt this to your NAS as required.
  • U1 is the MAX3232. Note that pin 15 (GND) and pin 16 (VCC, 3.3V) are not visible, but have to be connected nevertheless.
  • CONN1 is the male D-SUB interface with 9 pins, where pin 2 is RX, pin 3 is TX and pin 5 is GND.
  • The five capacitors are called C1 to C5.

Soldering

The following picture shows the soldering side of the board, to give you an idea how to place the components and which copper stripes have to be connected.

Layout of the board
  • Red: The positions of the five polarized capacitors. Watch their polarity!
  • Green: The arrows show which copper stripes have to be connected. You can do that either with small wires on the component side or with a soldering bridge on this side.
  • Violet: A short wire on the component side to connect pin 2 of U1 with the positive side of C1.
  • Blue: A longer wire on the component side to connect C2 at U1 pin 6 with GND.
  • Yellow: Here you will solder seven long wires. Three of them (2 RS232, 3 RS232 and GND) connect to the D-SUB RS232 interface while the other four are for the serial header in the NAS. Note that one hole (labeled GND) is used for both and gets two wires.

This is how the component side of the finished serial adapter looks like:

Component side
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