3 pins data communication Problem

[rlev]

Hi fellas,

I 've got the next problem :
I am designing a handheld device which should send commands serially to a host through a cable ( numbers of wires to be determinated ) de communication will be just one way from the handheld to the host.

The length of this cable link would be 100 mts ( 300 feet ) maximun.
I know I can use rs485 with :
- one wire for power ( maybe 12 vdc) to power up the handheld
- one wire for ground (GND)
-two more wire for the data ( +Data and -Data)

Four wire in total.

I wonder if exists another protocol that would allow me to use 3 wires only.
I read about the Dallas_Maxim 1-wire technology.......but I am not sure if
this could solve my problem.

The challenge is that 3 pin cables and 3 pins connector ( audio ) are really cheap and my company is trying to use 3 pins as its standard.

Any help will be appreciated.

 

[akouz]

RS485 is not a protocol
You can use RS485 receivers and transmitters for unbalanced single-wire link. Use just one output of RS485 transmitter, and one input of RS485 receiver. Apply +1.2V dc offset to another input of the RS485 receiver.

 

[IanP]

Akouz come up with good idea except you will still have problem with how to controll transmission/reception pin on the RS-485 transceiver.
Some years ago I was in need of reading RS-485 device trough my laptop and this resulted in a circuitry that I have to draw properly and pass on to you later. Basically it is 3-transistor circuit that from output looks like "open collector", input accepts TTL/RS-232 and the third transistor controls transmission/reception.
I will draw it later today so you will have a look at it.
And the most importat thing is that it works with up to almost 300m of cable and any baud rate !!!

 

[IanP]

Attached is the scanned circuit (not very good resolution)
Now, use only line A, this is your pin 1, pin 2 = +12V, pin 3 = 0V and your task is solved.

 

[rlev]

Thank you guys
I really appreciate your help.

keep the good work going.

rlev

 

[arcaphill]

Why not use RS232???

 

[eziggurat]

RS232 is not suitable for long distance communication.