Extending serial port cable with ethernet cable

[Ignoramus28229]

Anyone know whether a (9 pin) DB9, RS232 serial port cable can be
extended using 8 conductor Ethernet cable?

I looked at some schematics and was confused as to whether all 9
conductors are used in a serial cable, or one could get by with
less. If not, then I could have a shot at doing it, right?

i

 

[Tim Auton]

Anyone know whether a (9 pin) DB9, RS232 serial port cable can be
extended using 8 conductor Ethernet cable?

I looked at some schematics and was confused as to whether all 9
conductors are used in a serial cable, or one could get by with
less. If not, then I could have a shot at doing it, right?

It's not typical for devices to use all the RS232 lines (though some
do). Often just TX and RX are used, with RTS/CTS next most common in
my experience. You need signal ground too. If you have a specific
device in mind you could scope out the cable and see which lines it's
using.


Tim

 

[Spehro Pefhany]

It's not typical for devices to use all the RS232 lines (though some
do). Often just TX and RX are used, with RTS/CTS next most common in
my experience. You need signal ground too. If you have a specific
device in mind you could scope out the cable and see which lines it's
using.
Tim

Yes, you can also buy adapters that allow you to allocate the lines
whichever way you want (the modular connector has short leads with
poke-home female terminals for the DE9). It all fits inside the
housing when you're done. They are not expensive.


Best regards,
Spehro Pefhany

 

[Rich Grise]

Anyone know whether a (9 pin) DB9, RS232 serial port cable can be
extended using 8 conductor Ethernet cable?

I looked at some schematics and was confused as to whether all 9
conductors are used in a serial cable, or one could get by with
less. If not, then I could have a shot at doing it, right?

Eight is enough. For a quick-n-dirty cheater, you only need
3 wires; but if you want to do it "right", then just use one
pair for TXD/RXD, one pair for CTS/RTS, one pair for DSR/DTR,
and the other pair for signal ground and frame ground; I'm
not sure if you're supposed to strap those two together or
not. The other line in an "official" modem thing is RI, "Ring
Indicator", which you probably won't be needing. ;-)

Cheers!
Rich

 

[Ignoramus28229]

Eight is enough. For a quick-n-dirty cheater, you only need
3 wires; but if you want to do it "right", then just use one
pair for TXD/RXD, one pair for CTS/RTS, one pair for DSR/DTR,
and the other pair for signal ground and frame ground; I'm
not sure if you're supposed to strap those two together or
not. The other line in an "official" modem thing is RI, "Ring
Indicator", which you probably won't be needing. ;-)

Kewl!!! Soldering time tonight! This way, I could connect my linux PC
(running VMWare) in the basement to the serial port of the controller
in the garage. Yes, the ring indicator is definitely not needed.

i

 

[Tim Auton]

Eight is enough. For a quick-n-dirty cheater, you only need
3 wires; but if you want to do it "right", then just use one
pair for TXD/RXD, one pair for CTS/RTS, one pair for DSR/DTR,
and the other pair for signal ground and frame ground; I'm
not sure if you're supposed to strap those two together or
not. The other line in an "official" modem thing is RI, "Ring
Indicator", which you probably won't be needing. ;-)

Judging by the OP's response elsewhere in the thread there may be a
non-trivial cable length. Putting TXD and RXD on one twisted pair will
maximise the possibility of crosstalk. TXD and RXD are likely to be
the busiest lines, so, if they must be paired with data lines at all,
they should surely be paired with the least busy lines - DSR/DTR
probably. On the other hand, perhaps that would just make any
potential problems less frequent and thus harder to diagnose


Tim

 

[Rich Grise]

Judging by the OP's response elsewhere in the thread there may be a
non-trivial cable length. Putting TXD and RXD on one twisted pair will
maximise the possibility of crosstalk. TXD and RXD are likely to be
the busiest lines, so, if they must be paired with data lines at all,
they should surely be paired with the least busy lines - DSR/DTR
probably. On the other hand, perhaps that would just make any
potential problems less frequent and thus harder to diagnose

D'oh! Of course you're right. I had the differential TX/RX pairs in
mind from cat5. I'd trust your arrangement before mine, now that I've
been clued up.

Thanks!
Rich

 

[Rich Grise]

D'oh! Of course you're right. I had the differential TX/RX pairs in
mind from cat5. I'd trust your arrangement before mine, now that I've
been clued up.

Or use half-duplex. ;-)

Cheers!

 

[[email protected]]

Anyone know whether a (9 pin) DB9, RS232 serial port cable can be
extended using 8 conductor Ethernet cable?

I looked at some schematics and was confused as to whether all 9
conductors are used in a serial cable, or one could get by with
less. If not, then I could have a shot at doing it, right?

I remember an extension cord of ~6-8 meter causeing trouble at 38400 bps many
years ago. So you could use it as a "test". Ie use a 20m cat.5 cable, and see
if you can transfer between 38400.. 115200 bps without problems.

Anyway considering the nature of rs232 & cat.5 use the pairs like this:

1-2 TXD + GND Pay attention to numbering scheme..
3-6 RXD + GND
4-5 CTS + DSR
7-8 RTS + DTR

RIng, CarrierDetect dropped.

You get a full wireing. Meaning stubborn applications that insist on esp
dsr/dtr will work. Guess serial unix login will particulary benefit.
(no signal -> logout).

And in respect to interference and such. The two most important and sensitive
signals will be the most protected. However cat.5 have an impedance of 110 ohm.
And rs232 have undefined impedance .. or at least two, depending on if a '1'
or '0' is sent.

Oh and don't forget the dreaded DTE/DCE difference in wireing. Some are DTE to
DTE, others DTE to DCE etc..

http://www.hardwarebook.net/connector/serial/serial9.html
http://www.hardwarebook.net/connector/network/ethernet10baset.html

 

[Paul Hovnanian P.E.]

Anyone know whether a (9 pin) DB9, RS232 serial port cable can be
extended using 8 conductor Ethernet cable?

It depends on what sort of handshaking lines your application requires.
All 9 might be required, but many applications use fewer. 3 is the
minimum for bidirectional communications, 2 for a single direction.

I looked at some schematics and was confused as to whether all 9
conductors are used in a serial cable, or one could get by with
less. If not, then I could have a shot at doing it, right?

i

How far are you trying to extend the cable?

 

[Joe Soap]

It depends on what sort of handshaking lines your application requires.
All 9 might be required, but many applications use fewer. 3 is the
minimum for bidirectional communications, 2 for a single direction.

You need to connect Gnd, TxD and RxD. Many applications need RTS and CTS,
some use DTR and DSR. A very few use DCD.

I have never used RI (= Ring Indicator) in any design, over many years.

 

[Paul Hovnanian P.E.]

You need to connect Gnd, TxD and RxD. Many applications need RTS and CTS,
some use DTR and DSR. A very few use DCD.

I have never used RI (= Ring Indicator) in any design, over many years.

This is usually the case. On the other hand, my modem uses them all
(including RI) so it really depends on what the OP is up to.

If its a new system being designed, its easy to eliminate signal lines.
but if the goal is to make up some serial cables that will work in any
case, Cat5 cable won't cut it. In keeping with usenet etiquette, the OP
did not give us sufficient info to formulate a correct answer. ;-)