help to build a "NULL MODEM" Cable !!!

J

jpmm

Regular Member
hello there.
i a bit confused regarding to the creation of the "NULL MODEM" cable. after reading the ZackyFileS's manual of "How to construct a NULL MODEM cable"
i came to the conclusion that the best option for me would be to modify an existent commercial cable. therefore and because I intend to use a notebook (parallel port) to communicate with the receiver (by the way, i intend to use the receiver Mirage sci900s - fig.3 ) could someone confirm if the cable i should use is the "Serial (RS232) null modem cable (DB9-DB25)" as in the fig.1?

nullmodembig.jpg

fig.1

my other question is as follows:
so, using the fig. 2 , i should connect the DB25 Male (the one bellow) to the notebook and the DB9 Female (the one above) to the receiver (fig.3), right? acording to the manual i must use a DB9 male instead of the DB9 Female and swap the wires, right?

db9-db25-null.jpg








fig.2

but then, how do i connect a DB male to the reciever, which as a RS-232 male (fig.3)?

recep_sci900_b_l.jpg


fig.3

or, am i confusing everything?
if that is the case, sorry for my ignorance but if someone could clarify this for me it would be much apreciated.

jpmm
 
Right, here's what I think...

The cable you have there looks to me as though it's a Laplink type cable. The female 25 pin ends are for connecting the parallel ports on two computers together so that you can transfer files.

You can also do this via the serial port and this is what the two male 9 pin ends are for. You CANNOT use a parallel port at one end and a serial port at the other as they use different protocols and wiring formats. There's even a possibilty of bunging 12 volts up the wrong pin and damaging something.

All is not dark though. You will probably find that the 9 pin to 9 pin portion is a null modem cable. So connect one end to your computer and the other end to the receiver and, providing you have obtained the right software, you should be able to control whatever you want - or at least permitted to do.

A historical note. There is/was a serial port which used 25 pins but, in order to identify it compared to the parallel port, it had the same reversed pattern as the 9 pin port. If you happen to have one of these then you will need to buy a 9 - 25 pin converter. I have, somwhere, 6 headed lead with 25 pin parallel, 25 pin serial and 9 pin serial plugs at each end which I used to use for Laplink.

Good luck

PaulR
 
The cable I have used has 9 pin receivers on both ends (as the top connection on fig2 two) Hope this helps.

That was with a full blown PC not a laptop (hope they are the same).

Also used a crossover Ethernet cable.
 
Do you want to create one because you cannot buy one?
If this is the reason, go for it, and pray you got it right. Because from what I have managed to find out if it is wrong you might end up with a Dreambox that neeeds to go back to Germany to its maker...
 
Seems like a lot of worry when you can pick up a 10 metre null modem cable for £3-£4 +postage on fleabay.
One tip I liked (from another forum - I forget where) was to cut a short null modem cable in two and let in a suitable length of CAT or multicored phone wire - as long as the colours in the original two halves are matched correctly at each end and the bare wires are properly insulated against shorting it doesnt matter what colour the joining wire is - and you dont have to worry about which pin does what. But it is easier and safer to buy a 10 metre cable methinks! :) :)
 
The very simplest null modem cable requres just 3 wires!

pin 1 to pin 1.

pin 3 to pin 2.

pin 2 to pin 3.

This connects a common earth, and swaps the data lines round as required. It also assumes flow control is entirely done in software, so xon/xoff must be correctly set on the device at each end of cable.

If using hardware flow control, more wires are required, and it does get more complicated, as there's no "standard" way of setting up RS232 harware, so the xon/off method is easiest and recommended!

(also see: http://www.nullmodem.com/NullModem.htm ).
 
Just wanted to draw your attention to the fact that you have answered a thread from 2004...:-DOH!
I am pretty sure the problem of the null modem cable has been sorted since (I hope) :)
 
But still relevant!
 
Yep!

Cant remember how I found this one but as you say T_G it has probably been sorted for jpmm a long time ago.

Hopefully the info on the thread might come in for someone with a similar problem - particularly as many sat receivers now need a pc/internet null modem link for certain channels.

Cheers spiney - I must try that one!

:) :) ;)
 
T_G said:
Just wanted to draw your attention to the fact that you have answered a thread from 2004...:-DOH!
Click to expand...

Just seen that myself. Funny. Maybe he's still trying to figure: so pin X on the male goes to pin X on the female. Uum, is that seen from the pin or solder side. :-DOH!

Admit it, who has ever soldered a SCART without the diagram in front of them. LOL.:-rofl2

My 2p worth. Unless you have a bench with RS232 male and female parts in abundance, a soldering iron, the 'right' cable, numbnut patience and a vice - follow digdogs latteral advice - like, go and buy one.

.
 
Well, as I said, if you can use Xon/Xoff then it's easy, as all other hardware signals (pin voltages) are ignored!

But, if you have to use hardware flow control, then it becomes non-standard, and some "fiddling" around may be required. Pins 8/20 (?) - another 4 wires - and maybe more!
 
Back
Top