MAX232 Basics for Students

MAX232 Diagram

The MAX232 is a hardware layer protocol converter IC manufactured by the Maxim Corporation. Commonly known as a RS-232 Transceiver, it consists of a pair of drivers and a pair of receivers. At a very basic level, the driver converts TTL and CMOS voltage levels to TIA/EIA-232-E levels, which are compatible for serial port communications. The receiver performs the reverse conversion.

Used in embedded microcontroller systems, and computers, this IC has been one of the most popular components in production for well over two decades. If you have a microcontroller circuit that requires communication through a serial port, then this is the chip to use. This is a versatile IC, which is one of those wonderful components that solve so many signal conversion problems.


Serial Port Protocol

RS-232 is a serial communication protocol defined by the EIA/TIA-232-E specification in 1962. The letters “RS” refer to Recommended Standards.

This protocol requires a voltage between -3 V to -15 V to represent binary 1, and a voltage between +3 V to +15 V to represent binary 0. For CMOS and TTL communication, this is incompatible since TTL uses 5 V to represent binary 1 and 0 V to represent binary 0. This chip therefore performs the necessary protocol conversion of the electrical voltage levels in both directions.

One interesting thing to note is that RS232 communication also consists of control signals such as request-to-send (RTS), data-terminal-ready (DTR), data-set-ready (DSR), and clear-to-send (CTS). Usually, for simple projects, it is only the transmit (Tx), and receive (Rx) lines that are converted.

Dual Drivers and Dual Receivers

As you can see, there are two drivers, and two receivers in the MAX232 IC package. This can be confusing for students, as it makes the chip appear more complicated than it really is, but it is actually very easy since for most applications we generally use only one driver and one receiver.

I tend to use pin 7, pin 8, pin 9, and pin 10 for most of my circuits because I am used to using those. The other driver and receiver are not used, and these extra circuits are very useful as spares.

Inverters, Doublers, and Capacitors

Since RS-232 requires higher voltage levels, the manufacturers thoughtfully incorporated switched-capacitor charge pump circuits within the IC to generate the required voltage levels. The doubler, as the name suggests doubles the voltage level to produce +10V, whilst the inverter produces the negative voltage supply of -10V.

The external support capacitors C2 and C4 are part of the voltage inverter circuit that creates the negative voltage. However, C1 and C3 are part of the voltage doubler circuit.

As you can see, the proper operation of this IC relies on the correct value capacitors connected to it. There are many versions of this IC and they all have different requirements for the external support capacitors.

Support Capacitors

Version Support Capacitors
  C1(µF) C2(µF) C3(µF) C4(µF)
MAX220 0.047 0.33 0.33 0.33
MAX232 1.0 1.0 1.0 1.0
MAX232A 0.1 0.1 0.1 0.1
MAX222 0.1 0.1 0.1 0.1
MAX234 1.0 1.0 1.0 1.0
MAX211 0.1 0.1 0.1 0.1
MAX242 0.1 0.1 0.1 0.1
MAX238 1.0 1.0 1.0 1.0
MAX233CPP None required

This package comes in different versions and each version requires different value capacitors. The table above shows some of the MAX232 versions and their external support capacitor requirement.

If you do not like the idea of using capacitors, then there is always the MAX233CPP, which does not require the support capacitors. However, be prepared to pay a little more for that chip. From an education perspective, it is better to use MAX232 with 1-µF support capacitors. For one, you learn a lot more that way, and it is cheaper if you already have some recycled capacitors.


Packages

The “N” designation in “MAX232N” indicates PDIP package style, which is through-hole technology and best suited for student use. This package mounts conventionally with the rest of the components. Ideally, you should also get a chip socket to facilitate the recycling of this IC for other projects, as it is an expensive component.

The “D” designation in “MAX232D” indicates SOIC package, which may be difficult for beginners who lack soldering skills to work with.

Students starting out will not have the necessary soldering skills to use other packages such as the SOIC surface-mount devices (SMD), and therefore these are generally not used.

Cost / Source

You can get these from many online shops on Google such as eBay, RS, Maplin, and Farnell.

Maplin has the MAX232CPE, which are the PDIP versions requiring four support capacitors of value 1 µF. Order Code FD92A. They also have the MAX233CPP, which does not require any support capacitors. Order Code NR45Y.

My Projects using the MAX232

M Bus Serial Cable circuit
ATMega32 UART PC Interface
Casio Serial Cable