TX & RX on single wire and half duplex communication in analog signalling

Thread Starter

LavanyaThumati

Joined Sep 22, 2020
25
Hi

Here I am not using any RS232 or RS485 protocols. I will transmit analog square signal of -12 to +12V and also receive the -12V to +12V signal. So I have a circuit which can transmit and receive the signal individually. 2 circuits are constructed the schematic is attached below.

The process is First setup-1 will transmit the signal for T1 time period then after T1 period it will not transmit it will be in idle state i.e. 0V for T2 time period, in this T2 time period, the setup receive the signal and transmit the signal to setup-1 for T1 period in T2 period only. After that the setup again transmit the signal for T1 period and Setup transmit the signal for T3 period .. so on it will continue.

In this testing i have faced a problem that when both setups are connected each other and while doing the above testing process , the oscilloscope was connected between 2 setups for observing the voltage.. here the voltage which should be -12V to +12V it was dropped to -6 to +6V ..

can anyone please suggest any solution to this issue i.e, voltage dropping issue
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KeithWalker

Joined Jul 10, 2017
1,912
It would be much easier to discuss your circuit if the components had designators.
I am assuming that you made your measurements at "SIGNAL 1" in your diagram. If so, I don't see why you expected the full +/- 12V to appear there. The op-amp driving the two transistors has negative feedback which will reduce the amplitude of the output. Try displaying the waveforms at other points in the circuit to see what is happening to the signal.
 

MrChips

Joined Oct 2, 2009
24,168
What you are attempting is no different from CDMA (collision detection multiple access) and CSMA/CD schemes used in 1-Wire, 2-wire, RS-422, RS-485, IRDA, fibre, RF, cellular, internet communications.

In essence, all transmitters must be disabled and only one device allowed to transmit. If two devices are transmitting simultaneously then you have to implement collision detection and priority schemes.
 

Audioguru again

Joined Oct 21, 2019
3,509
Your transmitter has a 23 ohms output and it feeds the receiver at the other end that has a 23 ohms input. Then you have a voltage divider. But the voltage being reduced to half should not matter since your receiving opmps (used as comparators) produce a much higher output voltage.
 

ronsimpson

Joined Oct 7, 2019
1,456
Reinventing the wheel:
Link to MAX3190.
There are some RS232 transmitters that have a shutdown or tristate pin. You could have many devices on the same wire. All receivers are on all the time but only one transmitter is on at a time. The software would have to be like RS485 where only one device is allowed to talk at a time.
 

Thread Starter

LavanyaThumati

Joined Sep 22, 2020
25
You need to implement a way to disable the output drivers, i.e. turn off the two push-pull output transistors.
Yes, I need to implement a circuit for disabling the drivers.
I have a thought to put a switch at output of transistor driver, so that the switch will open when it is not transmitting the signal and it will be in closed when it is transmitting. so that when setup-1 transmits, switch in setup-1 closed and switch in setup-2 will be open becoz it will be in receiving mode. When setup-2 transmits switch in setup-2 closed and switch in seup-1 is open it will be in receiving mode. Here a extra control need to be added for transmission control.

By this method we can prevent the connection between 2 transmitters so that the voltage cannot drop. Is this a correct method to do
If correct how to configure transistor as a switch to pass -12V to +12V square wave.
 

Thread Starter

LavanyaThumati

Joined Sep 22, 2020
25
It would be much easier to discuss your circuit if the components had designators.
I am assuming that you made your measurements at "SIGNAL 1" in your diagram. If so, I don't see why you expected the full +/- 12V to appear there. The op-amp driving the two transistors has negative feedback which will reduce the amplitude of the output. Try displaying the waveforms at other points in the circuit to see what is happening to the signal.
I have observed signal at output of op-amp(amplifier) or before push-pull transistors. The signal is -12 to +12V which was in the expected way.
 

MrChips

Joined Oct 2, 2009
24,168
I would try an analog switch such as a FET or CD4066 on the output transistor bases to ground.
Applying 0V to the base will turn off the output transistors.
 

Sensacell

Joined Jun 19, 2012
2,890
What are you trying to accomplish actually?

Single-ended one-wire coms can be accomplished in a much simpler way.

One needs to define the problem to be solved.

What data rates?
How far?
What is the application?
 

crutschow

Joined Mar 14, 2008
27,706
A ±12V square-wave is not an analog signal, it's digital.
It may be encoded with an analog signal, but that doesn't make it analog.
 
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