MAX232 Basics for Students

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. The drivers convert between TTL and CMOS voltage levels (TIA/EIA-232-E), which is required for serial port communications. Used in embedded microcontroller systems, and computers, this IC has been one of the most popular components in production for many years. If you have a microcontroller circuit that requires communication through a serial port, then this is the IC to use. It is very versatile, and one of those wonderful components that solve so many signal conversion problems.

Serial Port Protocol

The RS-232 serial communication protocol was defined in the EIA/TIA-232-E specification, in 1962, and 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) data 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, however 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 +10 V, whilst the inverter produces the negative voltage supply of -10 V.

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 their external support capacitors.

Support Capacitors

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 of 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 the SOIC package, which may be difficult for beginners who lack the required soldering skills. Students starting out will not have the necessary soldering skills to use other packages such as the SOIC, or surface-mount devices (SMD), and therefore these are generally not used.

Cost / Source

The commonly available IC is MAX232CPE, which is the PDIP version requiring four support capacitors of value 1 µF. There is also the MAX233CPP, which does not require any support capacitors, and you can usually find these from online shops on Google such as eBay, RS, and Farnell.

My Projects using the MAX232