HC-05 Bluetooth Module

There are quite a few variations of this product on the market.
The product is usually provided with a 6 pin right-angle male header on the board edge.
I'd like to ignore the pins labeled Vcc, Gnd, Tx, and Rx.
That leaves 2 -- one probably labeled "STATE" and the other labeled either "KEY" or "EN".
I believe there is a difference between these 2 variations of the board.
I'm interested in knowing about the HC-05 boards with a pin labeled "EN"!
What is it's purpose?
It cannot be to place the module into "CMD" mode as opposed to the "DATA" mode!
I say that because the units I have purchased with the pin labeled "EN" have a momentary push button on-board that is used to place the module into "CMD" state and it works on all of my boards.
SO, what does the "EN" pin do?
I have a schematic showing the "EN" pin connected through a resistor to the "CE" pin of the on-board 3.3 v LDO regulator.
I have never been able to get any different behavior out of the module by connected the "EN" pin HIGH or LOW!
What does it do???

I have one that does nothing. It's supposed to put the module in "sleep" or "low power" state but toggling the pin does absolutely nothing. I can still send serial data across the Bluetooth connection. The "STATE" pin gets pulled low when the module is connected to another bluetooth

Thanks for your reply.
I knew about the STATE pin. I have a BLUE LED attached and it lights up when I use my phone to connect to the HC-05 module. (Behavior as expected and advertised).
The schematic I have shows EN, attached through a 1K resistor, to the "CE" pin of the on-board 3.3V LDO.
The EN pin has no trace on either the top or bottom of the board.
Assuming the board is a 2 layer board, then, as you say, the EN pin goes NOWHERE (it's NOT connected)!
Perhaps the schematic was drawn to show what might be a possible use of the EN pin.
My modules have 2 LSI chips on them. One is certainly the radio chip (BC417).
The other looks to big to be an LDO regulator and I can't read the number beneath the gold colored check mark on it topside. I think it's an FPGA.
The LDO regulator is probably the chip marked 662X.
Since there is only 3 resistors and 4 capacitors on the board, I'm convinced the EN pin is N/C.
Besides, the 662X only has 3 pins so there'd be no place to connect the EN pin (with Vin, Vout & Gnd).

BTW; what program do you use on your PC (if you do) to control the command pin (#34) on the HC-05?
Is it Putty or Tera Term or do you use the Arduino IDE?

I've never had the need to modify the Bluetooth setting using AT mode. I'm just using it as a replacement serial connection for debugging instead of using jumper wires with an FTDI board.

I use Putty and RealTerm. Realterm allows you to send special characters like \n and \r, and also see them, which is handy when troubleshooting

Thanks for the info about your development environment (Putty & Realterm).
I have a bit of a problem I just recently (in the past2-3 days) encountered.
Perhaps you can shed some light.
I have been using a UNO (its a variation called "Infiduino") to do my current work with serial I/O.
I decided to replace the UNO with the ItsyBitsy M0 Express.
All of a sudden, I'm now in trouble with serial i/o. (Can't find SoftwareSerial.h).
Everyone seems to be advising me to abandon software serial in favor of hardware serial, but no one is saying anything (at least that I can understand) about how to do that.
The Adafruit advice on changing over serial from earlier boards to the M0 is all about the Feathers and appears to be quite complicated -- I read most of the tutorial.
I was perfectly happy with software serial (I don't need speed and I have no timing problems) but it seems like no one is using software serial on the M0 boards.
In my setup with the UNO, I use the Serial.print for debug and I was using Pins 10,11 for Rx, Tx with SoftwareSerial to talk to the HC-05.
Now it all is too complicated.
Do you know of anywhere I can get either example code or straight forward explanations of how to do serial i/o on the ItsyBitsy M0 Express? (I have no idea if its the same as one of the Feathers that seem so popular).

The M0 express is based on a different architecture than the UNO (AVR vs Cortex M0+). It actually has 6 SERCOMs (SPI, Serial, AND/OR I2C) so you really don't need the software serial since it's got the hardware for it. I'm guessing no one bothered porting the software serial to the M0 since it's got enough hardware.

Try looking at how the Arduino Zero does serial. It's basically the same thing as the ItsyBitsy M0

Thanks for the tips. I'll check out the Zero.
I keep forgetting about these controllers having different architectures.
I usually just look at the MCU as a processor that should run my programs but I realize since I'm probably going to be looking at some low-level stuff, I can't view them just as processors that run code.
I read Lada Ada's "Using ATSAMD21 SERCOM for more SPI, I2C and Serial ports" but it's NOT written just to show how to use a serial port on the M0 boards.
Instead, she is explaining the whole story behind the processor internals -- good to know and I'm glad she wrote it, but at this particular time, I only needed some minimal info.
Turns out, all I had to do was use Serial1 in place of my SoftwareSerial port and it all worked! Serial1 uses the Pins 0 & 1 (the Rx & Tx for the Serial.print function typically used for debugging.
Now, I still use Serial.print to debug and it still goes to the serial monitor in the IDE and my data stream uses Serial1.
If I ever need more serial ports I'll have to struggle with Lady Ada's document.
However, I still think it's worthwhile to have the SoftwareSerial stuff available for the M0 for at least 2 reasons:

  1. Not everyone needs high speed (at baud rates less than 19200, it's probably OK in most cases).
  2. Older code would work as-is and that has value!

In the early 1960's, the US Air Force Strategic Air Command and Control System (Project 465-L) had 3 numbered Air Force headquarters connected with SAC HQ in Omaha with dual 1200 baud lines and it worked very well! We thought it was very fast.

Thanks again for your help -- it's much appreciated.

I work in IT/Comm Infrastructure for some very special customers and I had the privilege of designing and building a modern communications network that can support the transport of 72 baud serial...

My guess is the "very special customers" are typically identified by 3 letters! :o

When sht hits the planetary fan, you only need a couple of code words, to make more sht to hit the planetary fan, hence the very low baud rate but far reaching...