Help with this circuit

[SeanHatch]

Hello, I am building a monitor interface for a freescale microcontroller, and I have some questions about the schematic.

The schematic can be found here:

**broken link removed**

on page 165.

First of all, I'm looking to buy most of my parts from digikey, but they don't have one of the ICs in the schematic. Since the uC is a motorola thing, all the ICs are motorola too, but I figure there has to be a replacement for it. Here is the datasheet for the missing IC

http://www.datasheetcatalog.com/datasheets_pdf/M/C/1/4/MC145407.shtml

Also, on the far left of the schematic, you see some arrows marked 2,3,7. DB-25 is written above this. I don't know what this means. I assume it is the serial connector or something, but not sure.

Finally, in the center of the circuit you see the 6Mhz oscillator. You can either select that or that other little circuit (see note three). I'll have to build the 6MHz oscillator right? You can't just pick one these things up can you? and for the other 6mMHz option, what is that circuit symbol in parallel with the 10M resistor?

I know this is vague, but any help is appreciated. Thank you

 

[Papabravo]

The generic replacement for the MC145407, 5V only RS-232 Driver/Receiver chip, from Maxim is the MAX232 family along with more variations in terms of capcitor values, number of transmitters and receivers, and static discharge immunity, than you can shake a stick at. Linear Technology also has some parts like this.

On a DB-25 serial interface connector pin 7 is ground. Pins 2 and 3 are Transmit Data and Receive Data. The problem with copying this circuit is that regardless of weather your connector is a DB-25 or a DB-9 serial ports have all but vanished from modern PC's

Good news. You can buy a 6MHz. oscillator. It has the same footprint as an 8-pin DIP or a 14-pin DIP. They come in a little metal can. Hook up 5 Volts and GND, add a bypass capacitor and your oscillatin' to the oldies.

The symbol in parallel with the large value resistor is a quartz crystal. I think you need an AT-cut parallel resonant. Read the documentation carefully.

One other point. I see a symbol refered to as Vdd and I also see an explict supply labeled 3.3V. Are they the same, or will they be different. This is a really important point.

 

[SeanHatch]

well thanks alot. very nice info there.

VDD should be about 5V. That's the operating voltage for the uC. Since I'll be using batteries though, I could see it slipping down to 3.3V sometime or another. Will this cause damage or just malfunctioning?

Also, with the MAX232 IC, you mean internal transistors and capacitors right? I will not have to change the values of the capacitors between the pins, will I?

Thanks alot of the help

 

[Papabravo]

The MAX232 is the generic name for a family of RS232 drivers and receivers that numbers several hundred different parts. The concept is at least 20 years old. What you have to do is get a copy of the datasheet, from:
**broken link removed**

read it very carefully and follow the recomendation for the number, value and type of capacitors. When the chips first came out they used large expensive electrolytic capacitors. The newer chips (MAX3221 for example) use smaller non-polarized ceramic capacitors.
https://datasheets.maxim-ic.com/en/ds/MAX3221-MAX3243.pdf

Reread my original paragraph carefully. Inside the MAX232 are two(2) RS232 Transmitters(not transistors), and two(2) RS232 Receivers. The capacitors are external to the chip and implement two different circuits to generate voltage levels appropriate for RS232 serial communication. One circuit is a charge pump voltage doubler which makes +10Volts from +5 Volts. The other circuit is an inverter which makes -10Volts from +10volts. RS232 levels with these chips swing between -10 Volts and +10 Volts. There is a no mans land from -3 Volts to +3 volts where the logic level is neither a "mark" or a "space". It is undefined.

The purpose of a transmitter is to take a logic level signal in the range [0..5] Volts and convert it to an RS232 signal in the range [-10..+10] volts. The purpose of the RS232 receiver is to take an RS232 signal in the range[-10..+10] and convert it into a logic level signal in the range [0..5]

As to running this thing from a battery you'll want to have a low dropout voltage regulator between the battery and the board so that as the battery voltage drops the regulator voltage stays up until the regulator "Drops Out".