RS232C Voltage standards (SoftwareSerial vs RS232 shield)

Hi Guys,

I have an Arduino Nano which talks happily to my various devices. I have a Synscan v3 Telescope handset that I’d like to interface to, but I am unsure of the exact protocol or voltages.

The purchased PC connections seem to offer USB->MAX232->6p6c connection via a variety of cables and adaptors. It sounds simple but all the PC cables on offer seem to be USB(MAX232)->DB9(m)->DB9(f)->6p6c.

It seems that if I use SoftwareSerial I should be able to take the RxTx pins out of the Arduino (say pins 10 & 11) and run them straight over a 6 core cable to the 6p6c socket on the handset. However…

The various sites I read seem to say that RS232C is +5v->-5v, so I’m not sure where the -5 volts comes from in SoftwareSerial. Would I be better using the RS232 shield, or is there no difference? It would be a bit of a squeeze to get a shield in, but I think there is just space.

Thanks for your time on this.

Regards

Steve

A MAX232 device is intended to interface\convert the logic 0 and 1 that softwareserial uses into the +-5V for the RS232 connection.

srnet:
A MAX232 device is intended to interface\convert the logic 0 and 1 that softwareserial uses into the +-5V for the RS232 connection.

And does SoftwareSerial not do the same thing?

srnet:
A MAX232 device is intended to interface\convert the logic 0 and 1 that softwareserial uses into the +-5V for the RS232 connection.

The MAX232 devices I have tested actually put out a bit over +- 8 volts on the RS-232 lined.

Paul

SteveBz:
And does SoftwareSerial not do the same thing?

No, thats why you use a MAX232.

Paul_KD7HB:
The MAX232 devices I have tested actually put out a bit over +- 8 volts on the RS-232 lined.

Paul

So should do you think I should use a voltage divider to bring it down eg 1k +2kOhms?

Regards,

Steve.

SteveBz:
So should do you think I should use a voltage divider to bring it down eg 1k +2kOhms?

Regards,

Steve.

To what end? Do you not understand one end of the adapter goes to the Arduino pins and that end is 5 volts. The other side of the adapter goes to the other RS-232 device which can be up to +- 25 volts. Google RS-232.

Paul

Or read the datasheet for the MAX232, where its made clear how what is needed to use it.

Paul_KD7HB:
To what end? Do you not understand one end of the adapter goes to the Arduino pins and that end is 5 volts. The other side of the adapter goes to the other RS-232 device which can be up to +- 25 volts. Google RS-232.

Paul

Hi Paul, Thanks for your answer. No, I didn't understand that the opposite end could tolerate up to 25V. The Synscan is labelled mostly as 232, but sometimes 232C. I thought that 232C meant 5 volts. Even if the voltage is 5V, does the receiving device need to be able to cope with 25 V, is that what you are saying?

I spent 3 days Googling RS 232 before I posted here. It's just not very clear anywhere. Or maybe it is clear in some places, but the clear statements are outweighed by the unclear statements in other places, and you don't know which to believe. For instance many sites say "make sure you get your wires connected correctly or you could damage the computer, the handset or both", but that is not true if both ends will tolerate up to +/-25v. (Obviously I'm not including the Arduino here).

So in summary, it looks as though I need to use the MAX232 to connect to the Synscan, whether or not it's +/-5v or +/-8v, but not the SoftwareSerial. I've just ordered the MAX232 shield for £2.13. Not bad at all.

Thanks.

Steve.

PS So you live in the middle of a desert. I imagine you have beautiful dark nights. Do you have a telescope?

srnet:
No, thats why you use a MAX232.

So I don't understand, what does the SoftwareSerial do, then? If the Arduino Nano UART contains a MAX232 to produce Tx/Rx at 5v. And you can use SoftwareSerial to connect to another Serial device (ie to mimic the action of the MAX232), I really don't understand what is different. (Although I have indeed ordered a MAX232). Does the SoftwareSerial just do 0-5v then? Rather than -5v to +5v?

Thanks,

Steve.

PS What an amazing thing to launch your own satellite. Will you do another one? We amateur astronomers are not very happy with Elon Musk's plans to fill the skies with his ones. But, I'd personally love to have a satellite telescope.

SteveBz:
So I don't understand, what does the SoftwareSerial do, then? If the Arduino Nano UART contains a MAX232 to produce Tx/Rx at 5v.

Why do you think the NANO UART has a MAX232 hidden away ?

The hardware UART in the ATmega processor in a Nano outputs\inputs to two digital pins. Softwareserial emulates the UART hardware with software and also outputs\inputs to two digital pins.

PS What an amazing thing to launch your own satellite. Will you do another one? We amateur astronomers are not very happy with Elon Musk's plans to fill the skies with his ones. But, I'd personally love to have a satellite telescope.

I do have a design for a LoRa based PocketQube, fairly confident it will work. If anyone has £20,000 or so to fund the launch I would be happy to oblige.

Quote: "Hi Paul, Thanks for your answer. No, I didn't understand that the opposite end could tolerate up to 25V. The Synscan is labelled mostly as 232, but sometimes 232C. I thought that 232C meant 5 volts. Even if the voltage is 5V, does the receiving device need to be able to cope with 25 V, is that what you are saying?".

It's been so many years.... But I recall the "C" update changes the allowed cable length from 50 ft to 100ft. maximum. The reason was you could now buy low capacity cables that would not degrade the square wave signal.

Paul