Need a cable pinout for a device with serial printer output

Started by Acy, Oct 26, 2005, 00:10

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I have a device that has a Serial printer output. I need that device to connect to a serial port on a PC and display the output in Hyperterminal.

Can't I just use a standard DB25 straight through serial cable?


No, you just can't use a straight DB25 cable, if the manual doesn't say that the plug is crossed.

If you use a straight cable on 2 standard plugs you will connect PC's Transmit pin with device's transmit pin. This way no device will hear each other.

If the device has only 3 contacts, marked Txd, Rxd, Gnd you will need to biuld a cable. Get from the pinout of the pc's serial port, and connect device's Txd to pc's Rxd, device's Rxd with pc's Txd, the ground signals together, and remember to connect DTR, DSR and DCD together to fool the pc in thinking he has another pc connected.

- PC's pin 1 connected with PC's 4 and with 6
- PC's pin 2 connected with device's Txd
- PC's pin 3 connected with device's Rxd
- PC's pin 5 connected with device's Gnd

other pins may remain unconnected.

If the device has a standard plug, you can look for a so-called serial crossover cable.

Bye 8)


Alex, thank you very much, that answers my question and half a dozen others I would have needed answered before I get this project finished!


Ok, I got the output into Hyperterminal. But I still have a problem...

The Device monitors the connection, and for one reason or another, it continues to think that it's not connected.

Just for an FYI, this is a very old Alarm receiver with a DB25 male Serial Printer output.

It continues to beep every 10 seconds notifying me that the printer has failed.

I'm using an application (other than Hyperterm) on the PC that just keeps a log of all the printouts. It has Software XON/XOFF Flow Control set, baud rates match etc... And I get a printout on it too!

Here is what I have pinned, and maybe someone can tell me what I am missing:

Receiver -------- PC

TxD ---------- TxD
RxD ---------- RxD
RTS -|
CTS -|
SG ----------- SG
DSR --------- DTR

Tried The following in different combinations in addition to the above:

DSR --|______ DTR

DTR ------------ CTS

I also tried the following pinnout:

Receiver -------- PC

TxD ---------- RxD
RxD ---------- TxD
RTS ---------- DSR
DSR --|______ DTR
SG ----------- SG
CTS ------------ RTS
DTR ------------ CTS

Which left me with a question while I was looking for pinnouts, What is SHELL? According to a pinnout I found, it looks like Pin 18.

So I left that one disconected, but the pinnout said:

SHELL ---------- GND

The Manual for the Receiver states that my DSR should be connected to "ALWAYS ACTIVE OUTPUT PIN"  (DSR is what I'm assuming is causing my Receiver to think that the PC is NOT connected).

Both of these devices, the Receiver and the PC should be considered DTE devices. (From what I understand, which isn't much).

And if it hasn't gotten complicated enough... I am using a Digi

So my DB25 goes to an Rj45 through the Digi to the virtual comm port on the PC.

All Comm settings match through the entire route Except the Digi doesn't have flow control setup because it's not neccessary (shouldn't be neccessary) I got all PC side pin definitions from Digi and they have been accurate for all of my other receivers.

If I haven't come off as a complete newb yet, please let me assure you that I have no idea what I'm doing.

Any help would be greatly appreciated.


Sweet! Ok... The software or the PC was (whichever it is) either is not sending a DTR signal, OR the Pin I have for DTR is wrong.

So what I did, was this: I connected DSR from Receiver to Shell Ground and it stops the buzzing.

Now the only issue with this, is that the Receiver will never buzz if the software stops working. But it will buzz if the PC shuts down or the cable gets disconnected. And the software should never stop running unless the PC crashes so I think we're good to go on that anyway. (Hasn't ever happened yet and it's been running 24/7 for over a year).

Anyway Thanks for getting me headed in the right direction Alex :)


Last serial printer I used worked with a null modem cable (DTE-DTE)
You can take a straight 7 wire serial cable and use a nullmodem adaptor or
make your own. There are many diagrams you'll find on google for null modem
cables.. Some will be only 3 wire with the CTS--RTS jumpered as well as the other controls pins jumpered at both ends.

I have to make lots of specialty serial cables for PLCs and Industrial workstation.  They will require jumper between specific pins so that the software knows there is a proprietary cable hooked up..

Good luck