But how can the collector be at +5v when the transistor is switched on? Won't it cancel out with the negative voltage provided by the charge pump?The emitter of T2 is held to +5V which means that it will drag the collector to +5V when it conducts and lets the collector float to the -ve voltage when it shuts off.
But how does it do this switching, I am confused
Thanks for your reply, but it actually works. RS-232 communications with this scheme at 1200 baud, have been tested many times, with communication of the device with PC, serial dial-up modems, and device-to-device. So it must work somehow.It doesn't !
View attachment 171973
Not only is it R2L but PNP upside down. bad form.
Who says this Rx/Tx even works?
D2 clamps any negative voltage on pin 2. That seems to be a bipolar voltage violation for the Tx option.
I see Rx from pin 2 but do not see any Rx from pin 3.
Conclusion: Not recommended.
This is how I would make it work.
Rev A
View attachment 171974
The DPDT are just 2 jumpers options
The analog Rx threshold for RS-232 is 2 diode drops or 1.3V but is bipolar for margin. Schottky diode for Rx, pump
RS-232 levels are:Hi,
it may work, but I guess not according specifications.
The circuit around C1 is a voltage inverter.
R3 pulls the signal to negative voltage (which is HIGH for RS232)
T2 pulls the signal to positive voltage (which is LOW for RS232)
I expect close to +5V, but only about -4V due to voltage drops.
D2 and D3 are used as voltage limiter for the RxD input.
I don´t know whether R1 is according specifications.
I don´t have the RS232 levels in mind. Often one sees threshold levels +3V and -3V. But I think this are the levels for inputs.
Outputs should have higher levels to get a noise margin and to care for voltage drop and ground bounce.
As said I don´t know the specifications, but I find the -4V critical.
Klaus
T2_emitter is +5VBut what I do not understand is how the transistor switching works. When T2 is energised, at it's collector presents +5v. But how does this +5v interact with the -5v of the charge pump circuit? Shouldn't they be eliminate and equal to 0v?
Thank you it is clear not to me of how the circuit works.Hi,
T2_emitter is +5V
T2_collector is TxD
T2_base is via resistor to microcontroller_TxD
So when microcontroller is HIGH:
then T2 is high impedance and the TxD voltage is determined by the negative voltage supply through R3 --> -4V
When microcontroller is LOW:
then T2 is low impedance (much lower than R3) thus the voltage is almost --> +5V
****
signal voltage:
Imagine there are two identical devices communcating with each other.
One (sender) is driving the signal (wire) via 1k to -4V.
the other end (receiver) is pulling the signal via R1 and D2 to -0.3V (estimated)
then the wire voltage will be about -2.15V.
Clearly in the "unallowed area" between -3V and +3V.
Thus my worries about the -4V, R1, R3 and D2...
It´s not the -4V at the negative supply that counts. It´s the-2.15V signal voltage at the wire.
Klaus
Yes, it´s just an estimation.It might be that the estimation of -4v is too aggressive. There are low drop diodes used and the current drawn is very little.
Sorry but the reversed upside down schematic against IPC standards threw a curve ball.Thanks for your reply, but it actually works. RS-232 communications with this scheme at 1200 baud, have been tested many times, with communication of the device with PC, serial dial-up modems, and device-to-device. So it must work somehow.
I am not sure I understand your description of why it doesn't work?
RC pump is adequateThank you it is clear not to me of how the circuit works.
It might be that the estimation of -4v is too aggressive. There are low drop diodes used and the current drawn is very little. At the input of the charge pump there is a 200KHz full swing 0-5v oscillator, so maybe the voltage drop is not that much. I will measure it, I am curious.
I would like to use the RS-232 circuit with another inverter (non-charge pump) that already produces -5v and avoids this drop you mention. Here it is. Do you think it will work? I do not see why not, but you may have another idea.
It works but poorly.
TTL is inverse (negative) logic to RS-232, and both have the same analog threshold.
Rx clamps negative voltage but does NOT invert, so it must be done in software.
The gnd symbol is non-IPC standard.
It is upside down to IPC stds.
Sorry but the reversed upside down schematic against IPC standards threw a curve ball.
The PNP is just an inverter with 1k to -V and collector pullup to V+
The Rx also needs an inverter as RS232 to TTL is inverted logic, so it had to be done in S/W
View attachment 171988
RS-232 allows +/- 3V for short connections, yet the Analog threshold is the same as TLL 1.3V but TTL data is inverted logic .
--- Updated ---
RC pump is adequate
So my list below is correct?Yes the UART is negative logic . "0"=5V, "1" =0V
But RS-232 inverts negative logic to become positive logic. Rx > 1.3V= "1", < 1.3V="0" They use large bipolar swings to add margin for ringing and crosstalk,
I suggest you read/use the MAX232 specs.
View attachment 171989
May be I misunderstand this sentence. In any case, the chip needs inverting RS-232 interface, RxD inversion is missing in the above AM7910 schematic.AM7910 has RS-232 negative logic interface
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