Help understanding RS-232 Transmit/Receive circuit

Sensacell

Joined Jun 19, 2012
2,852
Without looking too hard...

It's most likely a voltage level shifter, the RS232 standard requires both positive and negative output voltages, this circuit translates to TTL levels.
 

Thread Starter

flopsweat

Joined Nov 2, 2020
4
This is from a Pfaff 1475cd sewing machine. The left side is RS232 the right is the machine interface. I'd I'd love to ditch the circuit and replace it with usb (I can produce my own PCBs.) Iam familiar enough with 232 but not the machine (right) side of the circuit.
 

MrChips

Joined Oct 2, 2009
23,955
Pin-2 is received data (RX_in)
R1 and R2 is a voltage divider. D1 gives protection from negative voltages.
When RX_in is sufficiently high, enough to make the base of T1 greater than 0.7V, T1 (NPN) conducts, pulling Yellow to GND (Green). When RX_in is below a threshold voltage, T1 is turned off and Yellow is pulled high through T1 collector load resistor R4.

On the transmitter side (TX_out at Pin-3), R5 pulls the base of T2 (PNP) to supply voltage at White. Hence T2 is turned off putting the voltage at Pin-3 close to 0V via R2 and D2. Pulling Brown to GND (Green) turns on T2 thus raising the voltage at Pin-3 to the supply voltage via R7.
 

MrChips

Joined Oct 2, 2009
23,955
Help understanding what?
It would be helpful to readers if your thread title gives some indication of the subject matter.

Also, one thread at a time, please.
 

Thread Starter

flopsweat

Joined Nov 2, 2020
4
Google USB to serial cable.
Thanks for you circuit analysis. I have many usb to serial adapters. I don't know the voltage provided by white but will probe it in the next few days.

Sounds like brown and yellow are simply device rx and tx albeit with a different voltage from the PC (or usb adapter). So the circuit is mearly a level shifter?
 

Papabravo

Joined Feb 24, 2006
16,508
Thanks for you circuit analysis. I have many usb to serial adapters. I don't know the voltage provided by white but will probe it in the next few days.

Sounds like brown and yellow are simply device rx and tx albeit with a different voltage from the PC (or usb adapter). So the circuit is mearly a level shifter?
That's about it.
 

Thread Starter

flopsweat

Joined Nov 2, 2020
4
Ok now i get it. Mr. Chips, PapaBravo, Thank you both.

Its a charge pump like the one described here. https://www.sparkfun.com/tutorials/215
However the one i posted has more protection than the discrete circuit posted at the link. Once i verify the voltage i'll be replacing this old circuit (on a DB-25) with a USB serial.
 

MrChips

Joined Oct 2, 2009
23,955
D2, D2 and C1 is a "charge pump" in some sense. It takes negative voltage from the PC TX and charges C1 to a negative value. When Brown is logic high T2 is turned off and TX_out at pin-3 (PC RX) goes negative.
 
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