# COM Port Cable Confusion



## scilek (Mar 6, 2019)

I have been successfully connecting FreeBSD machines to Windows machines through the serial port and with the help of what I call "a serial COM port cable" and a USB-COMPORT adapter. I use putty since it is the simplest. On the FreeBSD machine, I have these lines present in the /boot/loader.conf  file:


```
console="comconsole"
comconsole_speed="115200"
```

This is the USB COM Port adapter that I use:
https://www.digitus.info/en/product...ssories/serial-and-parallel-adapter/da-70156/

Some time later – I believe due to wear – my cable to started to malfunction. I would have to hold it in a specific position to make it work. (Unfortunately I have lost it.) So I decided to get a new better cable and bought one of these:
https://www.digitus.info/en/product...a-cables-dsub-9-jack-to-jack/ak-610106-020-e/

But to my amazement, this cable does not work. Please forgive my ignorance, but what am I missing here? Isn't here only one type of serial port cable, you know, the "one-to-one" type? If there ARE other types of serial cable, what type should I buy to make this work?


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## SirDice (Mar 6, 2019)

scilek said:


> Isn't here only one type of serial port cable, you know, the "one-to-one" type?


Nope. There are indeed "one-to-one" cables but also so-called "null-modem" types. In order to connect two hosts (DTE in serial lingo) you need a null-modem cable, not a straight one-to-one. A straight one-to-one would connect the RxD of one end to the RxD of the other, same for TxD. You need to connect TxD to RxD and RxD to TxD, which is what a null-modem cable does. 









						Null modem - Wikipedia
					






					en.wikipedia.org
				




It's simple enough to see which is which if you have a multimeter, if pin 2 on one end is connected to pin 2 on the other end it's a straight one-to-one. If pin 2 on one end is connected to pin 3 on the other it's a null-modem.


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## scilek (Mar 6, 2019)

SirDice said:


> Nope. There are indeed "one-to-one" cables but also so-called "null-modem" types. In order to connect two hosts (DTE in serial lingo) you need a null-modem cable, not a straight one-to-one. A straight one-to-one would connect the RxD of one end to the RxD of the other, same for TxD. You need to connect TxD to RxD and RxD to TxD, which is what a null-modem cable does.
> 
> 
> 
> ...



I see. Just like the good old crossover cable... Thank you.


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## SirDice (Mar 6, 2019)

scilek said:


> Just like the good old crossover cable


Yes, exactly. Old RJ-45 ethernet wasn't auto-switching and you needed a cross-cable to connect two hosts directly (without a hub or switch). Modern RJ-45 ethernet cards can automatically switch the receive and transmit so these cross-cables aren't as common any more. Old school serial never had this automatic switching.


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## scilek (Mar 6, 2019)

SirDice said:


> Yes, exactly. Old RJ-45 ethernet wasn't auto-switching and you needed a cross-cable to connect two hosts directly (without a hub or switch). Modern RJ-45 ethernet cards can automatically switch the receive and transmit so these cross-cables aren't as common any more. Old school serial never had this automatic switching.



This new learning amazes me Sir Bedevere. Explain again how sheeps' bladders may be employed to prevent earthquakes.


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