[Sussex] DB9 to RJ45 - minicom?

[John Crowhurst]

On Sat, August 19, 2006 14:50, Steven Dobson wrote:
> This is correct.  A modern day twisted pair ethernet port (as apposed to
> the old BNC MDI or 15-pin AUI connectors) use different hardware
> configuration to send and receive data.  IIRC (I'm sure a better
> hardware guy here will correct) twisted pair uses voltage differences
> between a pair of wires to send the data signal.  twisted pari ethernet
> uses two pairs of wires, one for send the other for receive.

RJ45 is used for other purposes than ethernet. If you use T1 and E1 cards,
the cabling is STP RJ45 terminated cable. If you use standard UTP
crosstalk can and probably will occur on the member pairs.

Your standard ethernet cable pinout is:

TIA/EIA 568A
1 White and Green (TX+)
2 Green (TX-)
3 White and Orange (RX+)
4 Blue (Not used)
5 White and Blue (Not used)
6 Orange (RX-)
7 White and Brown (Not used)
8 Brown (Not used)

TIA/EIA 568B
1 White and Orange (TX+)
2 Orange (TX-)
3 White and Green (RX+)
4 Blue (Not used)
5 White and Blue (Not used)
6 Green (RX-)
7 White and Brown (Not used)
8 Brown (Not used)

With Power Over Ethernet, pins 4 and 5 carry +VE and pins 7 and 8 carry -VE.

T1/E1 RJ-48C (RJ45)

1 RX1
2 RX2
3 GND
4 TX1
5 TX2
6 GND
7 NC
8 NC

> Good old serial, at it's simplest uses only three wires.  One for
> transmit, one for receive and a common ground.  If it uses other wires
> these are used only for handshaking and can be configured to be ignored
> by the hardware.

RS232 (one of the standard justifying serial connections) gives a maximum
voltage of +25V to -25V. These days, +9V to -9V are more commonplace.

RS232 can use 3 wires and you can use software XON/XOFF flow control to
work the connection. A typical RS232 device however, makes use of the
extra pins for flow control (RTS/CTS, DCD/DTR/DSR) and with a modem the RI
tells the computer that the phone is ringing.

> While it may be possible to build a unit that could convert the
> electrical signals between the two standards that is only the start of
> the problem.  The electrical interconnect is just one of sevel in the
> OSI seven layer model[1] - the physical.

You run the risk of frying the ethernet controller seeing as the voltage
levels are 0.2V and 2V rather than -9V and +9V and possibly destroying the
rest of the motherboard in the process.

> Steve