# Tag Info

20

They're definitely required! For their function look at the block diagram You'll see that they're connected to the voltage doubler and voltage inverter. These create +10V from the 5V power supply and -10V resp. This is done by charge pumps. An oscillator will control the switches so that either S1 and S3 are closed or S2 and S4. When S1 and S3 are ...

16

It uses one charge pump to double the supply voltage, and the second charge pump to invert it. The idea behind the charge pump doubler is that capacitors are first charged in parallel, then they are switched such that they are connected in series. (Source of picture: datasheet for ICL232, which is similar to MAX232.) As an aside, I've seen hacks where +...

16

In short, the answer is yes. If you look through the DATASHEET, you can see that it is fine to use ceramic capacitors. If you go to page 10, and look at the 'Typical Applications', you will find some notes at the bottom. In case that wasn't clear enough, it is also stated in 10.2.2, that ceramic capacitors can be used. In fact, you will probably find ...

15

MAX232 does not work with 3.3V supply voltage. You should either use MAX3232 (which is 3.3V equivalent part of MAX232) or you should supply 5V to your IC. Besides that there is only one thing wrong with your circuit: You should connect (+) pin of C4 to GND not VCC. For those who misinterpret the circuit: Pins 2 and 6 are NOT the supply pins of the IC. They ...

9

It's not likely that Max232 is pumping out +/-7.5V or more without the capacitors, more that your terminal accepts the logic level voltage coming out of your pic controller. The RS232 spec allows for anything between +3 to +15 and -3 to -15. If the supply line to the Max232 is 5V then simply providing +/-5V for the max232 is acceptable to your terminal. ...

7

The "C" is a 1969 Specification Revision. It does not dictate 25V but is compatible with that. Today the Spec is Ref F to expand speed, timing and low voltage enhancements and is backwards compatible. The standard has not changed with a threshold that is the same as TTL or 1.3V = Vbe*2 but all receivers handle a wide range of Tx bipolar voltages. Can ...

5

From page 13 of the datasheet: The MAX220–MAX249 have two internal charge-pumps that convert +5V to ±10V (unloaded) for RS-232 driver operation. The first converter uses capacitor C1 to double the +5V input to +10V on C3 at the V+ output. The second converter uses capacitor C2 to invert +10V to -10V on C4 at the V- output. From Maxim's Charge Pump page: ...

5

First off if there is any opportunity to eliminate the USB-to-RS232 cable then do so. This would also allow you to get rid of the MAX232 as well. Instead use an FTDI type USB-to-TTL_Serial cable. You can get these for 5V targets as the TTL-232-5V cable or for 3.3V targets as the TTL-232R-3V3 from http://www.mouser.com. It only takes a simple 1x6 header to ...

5

The outputs and inputs of your circuit are labelled as a DTE connection: a Data Terminal. Your PC is also wired that way. But RS232 works on the basis of a DTE-style device connecting to a DCE-wired device: Data Communications Equipment, i.e a modem. To fix this, swap pins 2&3 in the connection so that Tx on one side connects to Rx on the other, and ...

4

Okay, for anyone that ever encounters something like this in the future: The issue was that there was leakage from the power trace into the serial trace, such that a 60Hz wave was appearing on the SPI line, which obviously would mess things up. I know the likelihood of this same fix applying to other people is small, but maybe it will at least give you ...

4

It looks like the opto-cable is resuing DTR and RTS as it's own power input pins. Your MAX3222 uses charge pumps to generate the high positive and negative voltages for signalling over RS-232. To me, if your cable doesn't draw a lot of current, the easiest thing to do would be to connect the positive supply (DSR) to V+ (pin 3) and the negative supply (RTS)...

4

A charge pump can be appropriate for making power supplies for low-power opamps. A MAX232 contains two charge pump supplies, which can be used for your purpose, maybe. The MAX232 will create a positive and negative voltage, but since those supplies are designed to power its internal circuitry, extra available current will be limited. If you don't load the ...

4

The main problem is the V+ voltage doubler seems like it's not working efficiently on your setup. I've used MAX232 for many years (in fact I work at Maxim) and you should definitely see about 9.5V at V+ under idle, no-load conditions. The V- voltage inverter is powered from the V+ voltage doubler, so get V+ working first. Is there anything else connected ...

4

For transmit/receive indication, you want the LED to light when the line is NOT at idle. This means you need the LEDs to be lit when the line is low, since this indicator will be connected to the digital logic side of the converter, not the RS-232 side. The other issue is that the LEDs will likely take more current to light well than the UART signals can ...

4

Figure 1. An oscilloscope trace of a byte transmitted by an RS232 link. Source: Wikipedia RS-232. Note a few things: Logic is inverted. A '1' is a negative voltage and a '0' is a positive voltage. The grey area shows the valid logic level voltages. It is > 3 V for a '0' and < -3 V for a '1'. I'm worried that MAX232'c Voltage level (±25) harms the LCD?...

3

If there is a MAX232 on the XBee development kit, you need to add a level converter to the micro UART as well. If there is a way to bypass the MAX232 and connect to the XBee at the micro voltage levels, you could connect the micro Tx to the XBee Rx (and vice-versa) directly. If you don't want to wire up the MAX232 (and externals) yourself, you can use a ...

3

It is probably the crystal on your target board. A 16 MHz rock probably can't generate a clock for 115200 baud exactly enough. Over a long enough continuous stream of characters, the clocks on the two devices will slide out of sync. Eventually, you will get a framing error and a bad character, and the devices will resynchronize on the next falling edge (...

3

For TTL serial (and other logic devices in general) connecting one output to two inputs is typically OK, but connecting two outputs together is a no-no. In this case with the USB side disconnected the TXO line will be driven high which is the normal idle state for TTL serial. Then the programming port won't be able to reliably drive the line low and even if ...

3

Firstly a MAX232 has both transmit and receive functions so a CD4066 won't work in any way as a receiver because the likely receive voltages it might see could be +/- 15V and this is way beyond the maximum power supply rating of 20V for this device. As a transmitter it could work because, if you look at the MAX232 spec, it only generates typically +/- 7V on ...

3

Please go to this very useful page from Analog devices to get the full story. Here's a snippet or two: - Protection Scheme 1 As described earlier, the EFT and ESD transient have similar energy levels, while the surge waveform has energy levels three to four magnitudes greater. Protecting against ESD and EFT is accomplished in a similar manner, ...

3

You can use one "MAX232 type chip" at each end and have proper RS-232 levels between UARTs. The MAX232 is not suitable for 3.3V operation, but there are other similar chips (eg. MAX3232) that will work properly from 3.3V (as used by the RPi). If your PIC is running from 3.3V and the distance is small and you have a common ground you may be able to ...

2

RS232 signal lines have fixed directions. If the arduino is talking to the computer, and the printer is talking to the computer, using the same cable, then I'm pretty sure you will need a null modem connection (RX / TX etc crossed over) between the Arduino and the printer. Also the printer may use hardware flow control. If so, you will to connect up RTS / ...

2

Definitely required. Your drivers won't output anything without them. In this answer I explain how the charge pumps work using these capacitors and also why it's not a good idea to use 10$\mu$F instead of 1$\mu$F.

2

I suspect if you try your continuity tester probes both ways round on the pins you will get different results. The pins are not shorted, they have different functions, and an internal protection diode in one direction.

2

Your question was way to long to read, but it seems you are having some problems with a MAX232-type of converter circuit. I have made what seems to be a very similar circuit using one of the TI variants of the chip, and even sell it as a product. Go to www.embedinc.com/products/rslink2 and you can see all the documentation, from a picture to the board ...

2

You say you scoped pin 3 of the DB9 and saw 0-5V. Pin 3 is receive data from the outside world, and is driven by your USB-to-TTL serial cable. The MAX232 receives RS-232 levels on pins 8 and 13, converting them to TTL levels on pins 9 and 12, respectively. The MAX232 receives TTL levels on pins 10 and 11, driving RS-232 levels on pins 7 and 14, ...

2

I'm assuming you're using TI's MAX232. Other than missing the bypass capacitor, your schematic seems fine. See below the Typical Operating Circuit from the datasheet for a comparison. Also, regarding Jim's comments, according to the datasheet, your C4 capacitor negative (short) lead (which corresponds to C3 in the circuit below) can be connected to either ...

2

Yes, you can, but the MAX232 is really not designed to create supply voltages. The output voltage is not regulated very well, and there is a lot of noise on the outputs, especially when the load is high, so you will probably have to use some filter capacitors. Oh, and you absolutely need to pay attention to max output current because the output voltage from ...

2

I suppose it's 16MHz crystal problem as mentioned in comment. You should get 3.7%@115200, not 4.5%. Hence maybe 0%@57600 instead of datasheet's 2.1% may be crystal inaccuarcy. Anyway I also cannot use 115200@16MHz against any "precise" serial device. No problem at Arduino 2560 board using 2560<->16u2<->USB chain because 2560 and 16u2 have the same ...

2

I didn't look at the code, but here are some possibilities: Lack of bypass cap on the PIC. Until you fix this, all else is irrelevant. Possibly not waiting for the UART to be ready before writing a character to it. Try sending just one character intead of a string. If that works, then this is a likely cause. Normally I'd add baud rate to this list, but ...

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