How to wire a 9 pin serial connector into a RJ11 4pin connector

You can definitely transmit data using just TX & GND.

Firstly, you want to hook up the ATtiny85 TX line to the FTDI RX line (yellow on the TTL-232R). Make sure that the USB adapter can handle 5V - I'm fairly sure even the 3.3V TTL-232R is 5V tolerant.

According to the example page for SoftwareSerial, you need to set the direction of the TX & RX lines in your setup function:

// include the SoftwareSerial library so you can use its functions:
#include <SoftwareSerial.h>

#define rxPin 2
#define txPin 3
#define ledPin 13

// set up a new serial port
SoftwareSerial mySerial =  SoftwareSerial(rxPin, txPin);
byte pinState = 0;

void setup()  {
  // define pin modes for tx, rx, led pins:
  pinMode(rxPin, INPUT);
  pinMode(txPin, OUTPUT);
  pinMode(ledPin, OUTPUT);
  // set the data rate for the SoftwareSerial port
  mySerial.begin(9600);
}

The baudrate will be 4800 in your case. The SoftwareSerial library doesn't seem to support CTS & RTS, so just make sure you aren't using them on the host software.

Check out the reference page for more details, where they talk about some potential timing issues which may be exacerbated if you're running at 1MHz using the internal oscillator on the tiny.

1. Note what Olin says - why do this? You may have a good reason but (as noted below) RJ11 is non standard for this purpose and you are inviting compatability problems.

2. There is NO RJ11 serial data standard. See examples below.

3. Advising whatyou want to connect to and why may make this task FAR easier.

4. Example at end of RJ11 serial data pinouts.

5. Brief comment below on finding unknown pinouts.

The answer to your question is given on the blackbox page that you referenced

• Pin your own modular adapter to connect your serial equipment.

The reason that they did not connect the RJ11 connector to the DB9 is that there IS no standard way available.

The reason that they left the wires uncommitted was so that you could connect them in the way that suits you.

If you do not know what the pinout you need is, who does? (Really). What are you connecting the RJ11 circuit to? What equipment? / model? / brand? Do they provide a datas sheet or connection diagram?

Worst case you may have some equipment that has a serial circuit on an RJ11 connector that you want to connect to. In this case, the very best method is to get the manufacturer's information. If this is not available then there are ways to proceed.

Finding the pinout of a 6 pin serial connector.

This is just an overview -m knowing the equipment brand and model or, failing that, type of equipment and intended use will help geatly.

• 6 wire connector.

• Power off. Do any wires make low resistance connections with device chassis? Label as possible grounds.

• Power on. Is any line at -5 to -12V wrt chassis ground. If only one this is probably TXD. Compare to diagrams below. Any clues?

• If more than one low line one may be CTS or RTS or similar. With scope on check lines on turn. Depending on equipment TXD may have data burst. RTS may toggle or play around during startup.

• etc.

More on this if needed.

Examples - these are useful mainly to show how thoroughly non standard the various arrangements are.

Dell RJ11 - one implementation

H2Ns page describing their RJ11 serial pinout and connections for interfacing to other equipment

H2NS's contribution to the fun.

Here's an interesting page from Unitronics they have a number of somewhat different "arrangements"

Here's a plug pinout from one of their several arrangements - it's notable for being different from but similar to the Dell arrangement above. TXD and RXD are the same and there's some other partial similarities.

• CISCO RJ45 (NB not RJ11) (from)

  1 n.c. <- 7 RTS bn

  2 DTR <- 4 DTR bn/wh

  3 TxD <- 3 TxD gn

  4 GND 5 GND bl/wh

  5 GND 5 GND bl

  6 RxD -> 2 RxD gn/wh

  7 DSR -> 6 DSR or

  8 n.c. -> 8 CTS or/wh