Convert 6551 To MAX232: A Comprehensive Guide

by Marco 46 views

Hey guys! Ever found yourself wrestling with outdated serial communication chips? Maybe you're staring at an old Motorola MC6809P Microprocessor system, complete with its trusty UM6551A ACIA chip and a Texas Instruments SN75188N/SN75189 setup, and thinking, "There has to be a better way!" Well, you're in the right place. This guide will walk you through converting a 6551 to a MAX232 circuit, making your life a whole lot easier. We'll break down the what, why, and how, ensuring you've got all the info you need to modernize your serial communication setup.

Understanding the Basics: Why Convert?

Before we dive into the nitty-gritty, let's chat about why you might want to make this conversion in the first place. The original setup, using the UM6551A ACIA (Asynchronous Communications Interface Adapter) along with the SN75188N and SN75189 line drivers and receivers, was a common solution back in the day. These chips did their job, but technology has marched on, leaving them somewhat… cumbersome. Think of it like comparing a vintage car to a modern one; both get you from point A to point B, but one offers way more efficiency and features.

  • The SN75188 and SN75189: These are RS-232 line drivers and receivers, respectively. They handle the voltage level translation needed to interface with RS-232 serial ports. RS-232, for those who might not know, is a standard for serial communication that uses voltage levels like +12V and -12V to represent logic signals. The SN75188 takes TTL (Transistor-Transistor Logic) level signals from the UART (like the 6551) and converts them to RS-232 levels, while the SN75189 does the opposite.

  • The UM6551A ACIA: This chip is your classic UART, handling the serial communication protocol. It takes parallel data from your microprocessor and converts it into a serial stream for transmission, and vice versa. It's the heart of your serial communication, but it needs those line drivers and receivers to play nice with RS-232 ports.

So, what's the problem? Well, the SN75188 and SN75189 require a dual power supply (+12V and -12V), which can be a hassle. They also take up more board space compared to modern alternatives. This is where the MAX232 comes in as a total game-changer. The MAX232 is a line driver/receiver that only needs a single 5V power supply. It uses internal charge pumps to generate the necessary voltage levels for RS-232 communication. This means fewer components, a simpler power supply setup, and a more compact design. Basically, it's like swapping a gas-guzzling engine for a hybrid – same functionality, way better efficiency.

Upgrading to a MAX232 offers several advantages:

  • Simplified Power Supply: Ditching the need for a dual power supply is a huge win. It simplifies your circuit design and reduces the overall cost.
  • Reduced Board Space: The MAX232 integrates the functionality of both the SN75188 and SN75189 into a single chip, saving precious real estate on your PCB (Printed Circuit Board).
  • Modern and Readily Available: The MAX232 is still widely available and supported, making it a reliable choice for new designs and retrofits.
  • Lower Power Consumption: Generally, the MAX232 consumes less power than the older SN75188/SN75189 combination, which is always a plus.

Diving into the Conversion: The How-To

Okay, let's get down to the fun part: how to actually make this conversion. We'll break it down into manageable steps, making sure you've got a clear roadmap.

Step 1: Understanding the Pinouts

First, let's take a peek at the pinouts of the key players: the UM6551A, the SN75188/SN75189, and the MAX232. Having a clear understanding of which pin does what is crucial for a successful conversion. You wouldn't want to accidentally connect the transmit data line to the receive data pin, right? (Trust me, it happens!)

  • UM6551A ACIA: This chip's pinout will depend on the specific package (usually DIP-28). You'll need to identify the transmit data (TxD), receive data (RxD), control signals (like RTS, CTS, DTR, DSR), and the clock and interrupt pins. Datasheets are your best friend here! A quick search online will usually provide you with a detailed pinout diagram.

  • SN75188N: This is the RS-232 driver. Key pins include the TTL-side inputs, the RS-232 level outputs, and the power supply pins (+12V, -12V, and ground).

  • SN75189: This is the RS-232 receiver. Key pins include the RS-232 level inputs, the TTL-side outputs, and the power supply pins (+5V and ground).

  • MAX232: This chip combines the driver and receiver functions in a single package. Key pins include the TTL-side inputs and outputs, the RS-232 level inputs and outputs, the 5V power supply pins, and the capacitor connections for the charge pump circuit. Again, consult the datasheet for the exact pinout for your specific MAX232 package (usually DIP-16 or SOIC-16).

Step 2: Mapping the Connections

Now, we need to figure out how to connect the UM6551A to the MAX232. This involves mapping the signals from the ACIA to the appropriate pins on the MAX232. Here’s a general guideline, but always double-check with the datasheets for your specific chips:

  • UM6551A TxD (Transmit Data) --> MAX232 RxD Input: The transmit data output from the ACIA needs to be connected to a receiver input on the MAX232.

  • UM6551A RxD (Receive Data) <-- MAX232 TxD Output: The receive data input on the ACIA should be connected to a driver output on the MAX232.

  • Control Signals (RTS, CTS, DTR, DSR): These signals are used for flow control. Depending on your application, you might need to connect these. If you're not using hardware flow control, you can often leave these disconnected. However, if you do need them, you'll need to connect the corresponding pins on the UM6551A to the appropriate pins on the MAX232. Remember that the signal polarities might be inverted, so you might need to take that into account when mapping the connections.

  • Ground and Power: Connect the ground pins of all chips together. Connect the 5V power supply to the appropriate pins on the UM6551A and the MAX232. Crucially, disconnect the +12V and -12V supplies that were previously connected to the SN75188 and SN75189!

Step 3: The Capacitor Conundrum

The MAX232 uses external capacitors to implement its charge pump circuit. These capacitors are essential for generating the necessary voltage levels for RS-232 communication from a single 5V supply. The datasheet for your specific MAX232 variant will specify the required capacitor values (typically in the range of 1µF to 10µF). Make sure you use the correct values and voltage ratings. Incorrect capacitor values can lead to unreliable operation or even damage to the chip.

  • Placement Matters: It's generally good practice to place these capacitors as close as possible to the MAX232 chip to minimize noise and ensure stable operation.

Step 4: Disconnecting the Old Circuitry

Before you start wiring up the MAX232, you'll need to disconnect the old SN75188 and SN75189 circuitry. This involves physically removing the chips from the board or, if that's not practical, disconnecting the relevant wires. Make sure you isolate the +12V and -12V power supplies that were used by the SN75188 and SN75189. Leaving them connected could cause damage or unexpected behavior.

Step 5: Wiring it Up

Now comes the hands-on part. Use wire jumpers or a breadboard to connect the UM6551A to the MAX232 according to the pin mapping you created in Step 2. Double-check all your connections before applying power. A wiring mistake can fry your chips, and nobody wants that!

  • Neatness Counts: Try to keep your wiring neat and organized. This will make it easier to troubleshoot any issues later on. Using different colored wires can also help you keep track of the connections.

Step 6: Power On and Test

Once you've wired everything up, it's time to power on your circuit and test it. Connect your system to a serial terminal program (like PuTTY or Tera Term) on your computer. Configure the terminal program for the correct baud rate, data bits, parity, and stop bits. If everything is connected correctly, you should be able to send and receive data between your system and your computer.

  • Troubleshooting Time: If things aren't working as expected, don't panic! The most common issues are wiring errors, incorrect capacitor values, or incorrect serial port settings. Go back and double-check everything. A multimeter can be your best friend for checking continuity and voltage levels.

Advanced Considerations: Beyond the Basics

Once you've got the basic conversion working, you might want to consider some more advanced aspects, depending on your specific application.

Flow Control

As we mentioned earlier, flow control is used to prevent data loss when the receiving device can't keep up with the data transmission rate. There are two main types of flow control: hardware flow control (using RTS/CTS signals) and software flow control (using XON/XOFF characters). If you're experiencing data loss or buffer overruns, implementing flow control might be necessary. This involves connecting the appropriate control signals between the UM6551A and the MAX232 and configuring your software to use flow control.

Baud Rate

The baud rate determines the speed of serial communication. The UM6551A typically supports a range of baud rates, which are set by configuring its internal registers. Make sure that the baud rate configured on your UM6551A matches the baud rate configured on your serial terminal program. Mismatched baud rates will result in garbled data.

Interrupts

The UM6551A can generate interrupts to signal events like data received or transmitter ready. If you're using interrupts in your system, you'll need to connect the interrupt output from the UM6551A to an interrupt input on your microprocessor. This allows your system to handle serial communication events in an efficient manner.

Custom Cables and Connectors

The RS-232 standard uses a DB9 or DB25 connector. You'll need to make sure you have the appropriate cable to connect your system to your computer or other serial device. If you're building your own cables, be sure to follow the correct wiring diagrams. Incorrect wiring can damage your equipment.

Troubleshooting Tips: When Things Go Wrong

Let's face it, even with the best planning, things can sometimes go sideways. Here are some common issues and how to tackle them:

  • No Data Transmission:

    • Check Power: Ensure both the UM6551A and MAX232 are getting the correct power supply voltage (5V). Use a multimeter to verify.
    • Wiring Errors: Double-check every connection against your pinout diagram. A single misplaced wire can cause havoc.
    • Baud Rate Mismatch: Confirm that the baud rate settings in your terminal program and your system's software are identical.
    • Capacitor Issues: Verify that the capacitors connected to the MAX232 are the correct values and are properly connected.
    • RS-232 Cable: Try a different RS-232 cable. Sometimes, cables can be faulty.

  • Garbled Data:

    • Baud Rate: This is the most common culprit. Double-check your baud rate settings.
    • Parity, Data Bits, Stop Bits: Ensure these settings match on both ends of the connection.
    • Noise: If you suspect noise, try adding decoupling capacitors near the power pins of the chips.

  • Intermittent Issues:

    • Loose Connections: Check for loose wires or connections. Reseat any connectors.
    • Overheating: Make sure the chips aren't overheating. Proper ventilation can help.

Conclusion: Conquering the Conversion

Converting from a 6551 to a MAX232 circuit might seem daunting at first, but with a clear understanding of the components and a methodical approach, it's totally achievable. By simplifying your power supply requirements, reducing board space, and using a readily available chip, you'll modernize your serial communication setup and make your life a whole lot easier. So, go forth and conquer that conversion! And remember, datasheets are your friends, double-check your connections, and don't be afraid to ask for help if you get stuck. Happy tinkering, guys! You've got this!