RS-422/485 to RS-232

[DSGarcia]

I have a single board computer with an RS-422/485 serial port that I would like to convert to RS-232. I do not wish to use an external adapter but would like to place the circuitry on an internal interface PCB. I do not need any handshaking lines and I actually only need the transmit line, but knowing how to convert the receive line also would be useful for another application.
Thanks,
Dale

 

[blueroomelectronics]

https://www.rs485.com/
Lots of RS422/485 to RS232 adapters with schematics.

 

[DSGarcia]

Bill,
Thanks for the link, but I was looking for something that I could include on my own interface board, not an external converter. At a bare minimum, I want to convert the RS-422/485 transmit signal to an RS-232 transmit line.
Thanks,
Dale

 

[ericgibbs]

hi DS,
Can you post a pic of the circuit, showing the area of the existing serial circuitry?

 

[DSGarcia]

hi DS,
Can you post a pic of the circuit, showing the area of the existing serial circuitry?

Eric,
The SBC will be purchased off the shelf and all I currently have is the spec that one of the two ports is RS-422/485. I simply want to convert the RS-422/485 transmit logic level to RS-232 to send data to a serial device (no handshake lines).

Thanks,
Dale

 

[blueroomelectronics]

Check the Cricket schematic on my site (old site). It has a simple 2 resistor converter for RS485.

 

[DSGarcia]

Check the Cricket schematic on my site (old site). It has a simple 2 resistor converter for RS485.

Bill,
I am not sure I understand the converter circuit. It appears that the RS-232 transmit and receive lines are connected together with a 1K resistor. Then the RS-232 receive line (I presume because it is pin 2 on the DB9 connector) is connected to the RS-485 B- signal. Is RS-485 not full-duplex?

Could you please explain this for me--it appears that if one does not use the RS-232 transmit signal, you simply connect the RS-485 B- line to the RS-232 receive line with no conversion. ???
Thanks,
Dale

 

[blueroomelectronics]

RS485 is a differential signal, TX & RX are on the same cable pair. This simple solultion only works for one device ie a single RS232 and a single RS485. Otherwise you'll need to build a more complex circuit like the ones on RS485.com

 

[DSGarcia]

RS485 is a differential signal, TX & RX are on the same cable pair. This simple solultion only works for one device ie a single RS232 and a single RS485. Otherwise you'll need to build a more complex circuit like the ones on RS485.com

Bill,
Just to make sure I have it right--I simply send the RS485 B- line out as the RS-232 transmit line. Is it really that simple (presuming I do not need a receive line)?

Thanks,
Dale

 

[DSGarcia]

I discovered that the voltage specs for RS232 are different than RS485 but I just had a thought that perhaps someone could comment.

Could I put a diode in-line with the RS485 output to provide only positive pulses and feed that into a normal RS232 line driver?
Thanks,
Dale

 

[3v0]

In an earlier post Bill sent you to a schematic that did it with resitors. Did you find the site ?
Go to https://www.blueroomelectronics.com/
and click on the old site link.

 

[mneary]

Tell us more about the SBC.
It has an RS485 chip; what kind? Are you willing to remove (unsolder?) it? Does your SBC have access to any negative voltages?

Tell us more about the "internal" adapter you want.
Do you want to build an adapter (board?) that sits inside the same box, or do you want to integrate it into your SBC?

Tell us more about the other serial device. Will it insist on true RS232 specifications (+/- 3 to 12V) or can it accept "TTL" (0,5V) as valid?

I'm thinking in terms of removing the RS485 chip and replacing it with a piggyback board with an RS232 driver. The RS232 transmit driver could be as simple as a couple of transistors or at worst it might be something like a MAX202, MAX232, etc.