#max232

MAX3232 RS232 to TTL Serial Port Converter Module with DSUB9 Connector

The RS232 Board is used for establishing RS232 communication with devices powered from 3 VDC - 5.5 VDC. It contains the MAX232 (RS-232 Transceiver) and a DSUB9 connector used for connecting to a PC or any other MCU.

Hi all !

Today we want to share some experimentation we made with an Epson tm-u375 ticket printer and arduino mega. We want to share some detailed instructions of how we connect it and make it work.

1) Cable

The Epson tm-u375 has a serial interface, with a DB25 connector. Like is serial, in the other end, it has a DB9 so it can be connected to a computer. Even if DB9 has several pins, the printer reduce this to 5 cables really, the wiring schema is the following:

db9 pin             db25 pin 1 CD   ---------->   6 CD 2 RX   ---------->   2 TX 3 TX   ---------->   3 RX 4 dtr  ---------->   6 dtr 5 gnd  ---------->   7 gnd 6 dsr  ---------->   20 dsr 7 none 8 cts  ---------->   20 cts 9 none

For some reason, pins 1 and 4 are unified to pin 6 in the db25, and the same happens with pins 6 and 8 that are unified on pin 20 in db25

2) Connecting to arduino

After you have the cable done and working (test it with a PC first if you can), it’s time to hook it up to the arduino. Unfortunately, RS232 signals are not compatible with the arduino directly, why it uses 10V/12V and the board can’t handle those voltages in the inputs / outputs pins. For this reason, we need a level shifter. The most common, is the MAX232, and it has several ways to connecting it. Thanks to this page i could find a working schema, and is the following:

And to see it in more details of how to actually connect this to the arduino:

Arduino Side

Pin 11 of max232 to TX of arduino

Pin 12 of max 232 to RX of arduino

Pin 10 to some PWN pin

Pin 11 to some PWN pin

DB9 Side

Pin 1 y 4 of DB9 goes togheter to Pin 7 of max232

Pin 2 of db9 to Pin 13 of max232

Pin 3 of db9 to Pin 14 of max232

Pin 6 y 8 of db9 togheter with Pin  8 of max232

Pin 5 of db9 to GND

3) Time to print boys !

If you wired everything correctly, now is time to print, thanks to this thread in the arduino forum, i we have an example code to use:

void setup () { Serial.begin(9600); //Epson default printer settings for baud rate } void loop() //looping sequence { printFromAtoZ(); delay(1000);                         //delay for 1 second } void printFromAtoZ() { unsigned char caracter = 'A'; Serial.write(0x1B);             //ESC POS command Serial.print('@');                   //ESC POS initialize followed after command for(uint8_t i = 0; i<=25;i++){ Serial.write(caracter++); } Serial.write(0xA);                    //Print and Line Feed from Buffer }

And voila ! it should be printing the A-B-C :D.

Troubleshooting

In case it doesn’t print, you must check every section, i recommend starting in this order:

1) Check the cable with a PC.

2) Check the wiring of the max232

3) Check the output voltages of the max232

The arduino has a built in USB-serial converter.  While playing with your arduino, this is an invaluable tool.  It is also the way that the arduino can be programmed without using the ICSP header.  But what if you want to pull that ATmega and put it on a custom board?

I asked that question because that's exactly what I want to do.  I want to make a small custom PCB for a single purpose.  The ATmega chip is a good candidate for this board (due to the number of I/O lines needed).  Since the ATmega has built in serial, and I wanted to enable serial communication anyway, I figured why not expose the build in serial and skip the ICSP header.

I ran into one major problem, everyone assumes you want to use a MAX232 (or equivalent) chip.  I don't have any laying around, and they consume a relatively large PCB footprint.  Routing the traces could also prove difficult (I didn't really examine the option).  I wanted cheap and small footprint that could easily be routed.  SparkFun has an excellent level shifter (https://www.sparkfun.com/products/133) based on the voltage stealing principal.  The sketch I created above is based on that level shifter.  I want the functionality on-board, not via a daughter board, so while the kit is useful, it's not useful here.  I examined their schematic and the board itself to break apart the various functions.

The capacitor is wired backwards on purpose.  The diode allows the capacitor to charge with a negative voltage that can then be used on the TX side.  The LEDs are purely informational and they are wired to light when the TTL levels go low.  The transistors are used to invert the TTL signals to RS232 "compliant" signals.  I tested this with a USB-Serial adapter cable and it worked.  My PC doesn't have a built-in serial, so I don't know if it will work with a true serial port.