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  1. #1
    6146-B Billy T's Avatar
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    Exclamation Null Modem Connections, DB-25 to RJ6

    Hi Team

    I'm struggling to diagnose a communication problem with two of my instruments. they download data via an RS232 connection with DB25 to DB9 adapter at the computer end, and an RJ6 connector at the other (which goes to the instrument) and I think it may be a mistake I have made in reterminating the RJ plugs because I have two identical instruments giving the same result with two separate (rebuilt) leads. I used a proper professional tool to do the terminations so they are OK and a continuity test confirms that.

    When I initiate a download, the instrument display normally blinks at regular intervals, which I take to indicate that some kind of handshake going on. I had noted in the past that if a plug was not properly inserted that this flashing effect was absent. Now I get the same effect with both leads and both instruments, and it is asking a bit much to expect that both would fail simultaneously. I know that the DB-25 end of both leads are OK because I haven't touched them, and I have tried downloading on two different computers but with the same result.

    The unaltered DB25 connections are:

    1: Shield (not terminated at the instrument end).
    2: Tx (White)
    3: Rx (Black)
    4/5 linked
    6: Linked to 20
    7: Received signal line detect (Red)
    8-13, 14-19, and 21-25 are all N/C

    The RJ 6 plug connections (as I made them) are:

    From the plug end with securing clip facing down:

    6: Red, 4: White, 1:Black

    From the cable end with the clip facing down:

    1: Black, 4: White, 6: Red

    The data I need to download is for a medical facility so it is quite important, and I have emailed my contact at the US manufacturer but have had no reply. They ceased manufacture some years back and recently terminated their service facility as well. I suspect my contact has either been made redundant or retired.

    I am assuming that there is a convention for RS232 to RG Plug interconnection and I think the setup is called a null modem but don't quote me on that, I could be in cloud-cuckoo land. I have searched extensively on the web and found a variety of relevant sites and wikis but my problern is thet they are written in jargon suited to those with a good basic understanding, which I lack.

    If I do have the plugs right, both instruments may possibly have time-expired as they have a Dallas DS1286 watchdog inside (whatever that is) last replaced in 2006, but the originals lasted over 10 years so I'm doubtful that the current units are the cause.

    Any help on the connections would be greatly appreciated. This is one time that I'm hoping desperately that I made a mistake!

    Cheers

    Billy 8-{)
    Last edited by Billy T; 25-01-2012 at 12:08 PM.
    Some days it's not even worth chewing through my restraints!

  2. #2
    Computer "Specialist" Agent_24's Avatar
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    Default Re: Null Modem Connections, DB-25 to RJ6

    Can't you compare the wiring to the old cables\plugs?

    I assume it had a working plug on there at some point before you re-terminated the cables.
    Non-system disk or disk error. Replace and strike any key when ready.

  3. #3
    6146-B Billy T's Avatar
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    Default Re: Null Modem Connections, DB-25 to RJ6

    Quote Originally Posted by Agent_24 View Post
    Can't you compare the wiring to the old cables\plugs?

    I assume it had a working plug on there at some point before you re-terminated the cables.
    Both cables are custom builds by the manufacturer and are the only ones I have. I was quite certain that the first one (re-terminated several months ago) was working, but it is possible that I have only been using the second one since that time. I reterminated the second one by copying the first yesterday and it was after that when I found that neither work. In any event, both are currently identical, the DB25 end remains original on both, so that only leaves my RJ6 terminations. If it isn't them, the outcome doesn't bear thinking about!

    My present problem is that I can't find any (understandable) reference to null modem (serial print modem?) terminations for a 6-pin RJ plug. I am thinking it is the white TX pin that should be on 3 instead of 4 but I don't want to make matters worse.

    Cheers

    Billy 8-{)
    Some days it's not even worth chewing through my restraints!

  4. #4
    6146-B Billy T's Avatar
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    Default Re: Null Modem Connections, DB-25 to RJ6

    OK, I'm still working on this issue.

    As I see it, with just three conductors in the cable, I can only have data send, data receive, and a handshake/reference level of some sort. That looks like it might be DSR (Data Signal Received).

    The RS232/DB9 connector carries only TX (pin 2) RX (pin3) and a "Received Line Signal Detect" or "Received Signal Detect" RSD for short (on pin 7) which I assume to be the same as DSR. That is all that there is to pass between the instrument and computer.

    Since everything in the computer/data connection world is fairly standardised, I thought it would be a doddle to check my cable connections, however it is anything but! I've spent many hours reading pages of information, all of it contradictory or written in Afghani.

    In the end, it comes down to a simple question of "did I reverse connections 1 & 6 or have I moved the Rx line from 3 to 4?". The stakes are too high for me to just play roulette with the wiring, but I can answer the question in a heartbeat if I could only find a 3-wire null modem connection for a six pin RJ plug. Once upon a time you just skipped to the back of the DSE or Jaycar catalogues and it was all there, but that content is long gone from DSE, including the catalogue itself, and I fear it is slowly ebbing away from Jaycar.

    I'll have to do some other work for a while now, but any helpful comments would be appreciated. I've tried to describe the cable link as simply as possible.

    Still no answer from the US.

    Cheers

    Billy 8-{)
    Some days it's not even worth chewing through my restraints!

  5. #5
    Computer "Specialist" Agent_24's Avatar
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    Default Re: Null Modem Connections, DB-25 to RJ6

    I had a look in my DSE catalogs but can't find anything about RJ6 plugs and RS232.

    They have the pinouts, electrical specifications and how to connect two PCs together but nothing else. That was even going as far back as the 1982 catalog.


    You say: "The unaltered DB25 connections are:

    1: Shield (not terminated at the instrument end).
    2: Tx (White)
    3: Rx (Black)
    4/5 linked
    6: Linked to 20
    7: Received signal line detect (Red)
    8-13, 14-19, and 21-25 are all N/C"


    4\5 linked and 6\20 linked matches that of the pinout for a null modem cable, but DSE says pin 7 is Signal Ground - not sure if that is the same as "Received signal line detect" - doesn't sound like it unless I'm missing something


    Are you able to open the equipment up? Perhaps you can trace the connections of the 3 lines in the RJ6 socket and try to figure out what makes sense.
    Non-system disk or disk error. Replace and strike any key when ready.

  6. #6
    amateur expert dugimodo's Avatar
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    Default Re: Null Modem Connections, DB-25 to RJ6

    RJ6 doesn't appear to be a valid connector designation, RJ61 is an 8 pin connecotr and RJ11 is the common 6 way one use on phones - is it one of those perhaps?
    Does the equipment not have a manual with the pinout for the connection?

    This page http://www.lammertbies.nl/comm/cable...nics-rj11.html hase some info for an RJ11, but because RJ11 is not a a standard serial connector the manufacturers didn't necessarily use the same pinout.
    http://en.wikipedia.org/wiki/RJ11,_RJ14,_RJ25 has some general info about the RJ standards.

    I'm not 100% sure of this but I don't think you can hurt anything by wiring it wrong, so unless you're in danger of permanently damaging the cable couldn't you just do some trial and error? Although it's abit like picking a small combination lock.

    I'll see if I can find my old notes from my telecom Installation days when I get home, I had pinouts for all kinds of things.
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  7. #7
    Senior Member MushHead's Avatar
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    Default Re: Null Modem Connections, DB-25 to RJ6

    Your basic 3 wire serial connection has Tx, Rx and GND signals. If you know that the GND signal is going through OK, then you need to try swapping 2 & 3 - the usual mismatch is the transmit and receive. Pins 2, 3 & 7 on the DB25 become 3, 2 & 5 on the DB9. As you've noticed, there's not much of a "standard" pinout for anything outside the usual 2 connectors.

  8. #8
    6146-B Billy T's Avatar
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    Default Re: Null Modem Connections, DB-25 to RJ6

    Quote Originally Posted by Agent_24 View Post
    4\5 linked and 6\20 linked matches that of the pinout for a null modem cable, but DSE says pin 7 is Signal Ground - not sure if that is the same as "Received signal line detect" - doesn't sound like it unless I'm missing something.
    You have got me going now, I can't for the life of me find where I located the Pin 7 'received signal' reference and that is driving me crazy. I agree that it could be incorrect in this instance, because the original RS232 termination has never been altered so it is my ID of the conductors that is at fault. But what a poor choice of conductor colour, the cable has red, white, and black. I'd have coded it Red for send, White for receive and Black for ground.


    Are you able to open the equipment up? Perhaps you can trace the connections of the 3 lines in the RJ6 socket and try to figure out what makes sense.
    Yes, I can open them up, I have done so several times to modify the instruments, mainly for minor repairs like replacing the battery connectors. I also added sockets for external power, because it had memory capacity to record for over 15 days at 1 minute intervals (366 hours) but an internal battery life for about 3-4 days. Go figure!

    I have looked at it internally, but the board is double-sided and not much is visible, though I will certainly be giving it a go if I don't make progress on confirming the lead configuration. In the meantime I am working out the modem connections again, taking into account the need to use a 25pin to nine pin adapter for the computer connection.

    Cheers

    Billy 8-{)
    Some days it's not even worth chewing through my restraints!

  9. #9
    Systems Engineer Alex B's Avatar
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    Default Re: Null Modem Connections, DB-25 to RJ6

    http://logmett.com/index.php?/quick-...e-pinouts.html

    What you can also do is download a little program on the PC called putty, tell it you want to connect Via COM1 (It's most likely COM1), you should be able to see a data stream or at least be able to send some data to it.

    http://www.omnisecu.com/cisco-certif...-or-switch.htm those instructions should work for most things.

  10. #10
    Senior Member
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    Default Re: Null Modem Connections, DB-25 to RJ6


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