"Steve" wrote in
:

I think when they say standard RS-232 is means Data 1 to 1 and so on.
If you cross them, is it then a Null modem cable.

That is correct provided the correct pins a crossed.

Standard is straight through, the pins correspond. There should also be a
male and a female connector. Each lead in the interface is assigned a
purpose e.g. DTE pin 2 is TX Data DCE pin 2 is RX Data. Since the type of
interface (DTE or DCE) has different functions assigned to each pin, a
standard cable allows for example the DTE to TX on pin 2 and a DCE to RX
on the same pin 2. Therefore you do not require a crossover in the wiring.
However when you are interfacing a DTE to a DTE you now have a problem.
Firstly the connectors will not match a standard cable, which has a male
on the DCE end and a female on the DTE end. Two DTEs would both have male
connectors so the cable should have two female connectors. Also since they
are both DTE the both would try to send data on pin 2 and they would never
RX any data from each other. That is why you have to crossover pin 2 on
one end to pin 3 and vice versa on the other and so that the DTEs can talk
to each other. The same applies to a pair of DCE's but the connectors on
the cable should both be male. If the equipment has a 9 pin on one end
and a 25 in on the other it may under some circumstances require circuit
modifications to get them to work if some of the flow control leads are
required for the interface to work. This is because the are 16 more leads
on one end. That is when you get in "custom" cables. Fortunately most PC
applications usually only require pins 2,3 and 7 to work. IF you cannot
get it work using thise 3 leads, then you will require a more than basic
understanding of the inteface parameters or the devices you are trying to
interface.

The above is all predicated on the equipment you are using correctly
adheres to the RS-232 standard, something that is not always the case.



--
Panzer