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Hey serial-knowing friends: are all RS232 DB25 pinouts the same? Are any two DB9-to-DB25 adapters going to be identical? I have a DB25-equipped serial device I’m getting a USB interface for, but looks like most USB interfaces only come in DB9 flavour.

@misty I don't remember any unusual DB25 serial pinouts, and all the 9/25 adapters I have seen were the same (though it's not impossible there's some that are wired for null modem).

@misty

are all RS232 DB25 pinouts the same?

Short answer: Now-a-days, yes.

To be absolutely honest, though, almost but not quite; however, the incompatibilities generally only concern older equipment specific to Telco applications (things like T1 CSU/DSUs, etc.) which make use of all 25 pins.

For PC use, only the data and flow control pins are used, so the only real difference you'll find relates to voltage levels (e.g., Commodore Amiga uses 9V levels, while some PCs might use 12V), but even this will not cause issues in practice, since the EIA specs says hardware is to only look for "voltages larger than +/- 5V."

I've only seen one device for a PC which didn't work on my Amiga, and that's because it expected a full 12V supply. Replacing it with a newer model worked fine, though.

Are any two DB9-to-DB25 adapters going to be identical?

For general purpose PC computing, yes.

I have a DB25-equipped serial device I’m getting a USB interface for, but looks like most USB interfaces only come in DB9 flavour.

I can't think of any contemporary reason why this arrangement won't work. You should have good success with most commercially available options today.

@misty you often get the 9/25 adapter with a usb-232 cable, e.g. premiumcord

@misty 1) No they're not all the same, but effectively yes. It is possible, however, that not all DB-25-using devices have all of the pins hooked up; in fact, _most_ have no more than about half a dozen pins hooked up. Older devices sometimes do very strange things, but if it's after the mid 70s I doubt you'll see that.

2) The only difference might be whether the adapter connects the common hardware handshaking lines or not. Most will. Some will connect _only_ pins 2 & 3 and ground.

@misty TL;DR: There's a very high probability that any two serial ports will plug together with "standard" cables and adapters, the only _likely_ bugaboo is if one end requires hardware handshaking and either the cable or the other end does not support it. You can sometimes "fix" this with some creative jumpering.

@elb Thank you so much! This is really helpful. I found a copy of the pinout for my device, which specifies that it does use the handshaking signals and not just the data pins/ground, so I’ll need to make sure whatever I buy supports that.

@misty in particular if you want to use a USB serial adapter you'll need to verify that it has hardware handshaking. The physical 9 to 25 pin adapter is _probably_ fine.

@misty RS232 means 232 ways to define serial connection.
It's a joke but not so far from reality.
Be careful ☺️

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