https://en.wikipedia.org/wiki/Null_modem

Have you tried Wikipedia, has wiring diagrams for Null modem cables, and there is a number of null modem cable designs depending on handshaking.

TrashPanda wrote on 2023-03-02, 11:39:

https://en.wikipedia.org/wiki/Null_modem

Have you tried Wikipedia, has wiring diagrams for Null modem cables, and there is a number of null modem cable designs depending on handshaking.

yeah, it has the exact same diagram i used, but the pins are in reverse order which i suspect is the correct pinout. my cable is wired in reverse and google algorithms are crap?

stanwebber wrote on 2023-03-02, 11:48:

TrashPanda wrote on 2023-03-02, 11:39:

https://en.wikipedia.org/wiki/Null_modem

Have you tried Wikipedia, has wiring diagrams for Null modem cables, and there is a number of null modem cable designs depending on handshaking.

yeah, it has the exact same diagram i used, but the pins are in reverse order which i suspect is the correct pinout. my cable is wired in reverse and google algorithms are crap?

hmm are you sure its a null modem cable ?

stanwebber wrote on 2023-03-02, 11:31:

it clearly labels both ends db9 female which is the cable connector, not the male serial port (unless the normal convention is for non-pc devices). is this pinout really for the serial port which means the cable connector pinout would be the mirror opposite which means my cable is wired in reverse? (pin 1 is pin 5 and pin 6 is pin 9.)

You do know there are numbers printed on any decent DB9 connector, next to each pin, right? You are supposed to connect the numbers, trying to figure out what is mirrored or rotated on crappy internet diagrams will only get you into trouble.

If this is for DOS CTTY COM1, be aware that DOS needs to see DSR set or otherwise it will give you abort/retry/fail forever. If it's one of those modular cable ends it might be possible to open it up and modify it to connect DSR with CD or some other pin that is connected to DTR at the opposite end.

i re-did the cable correctly. i used the zoom lens on my camera phone and the pins were indeed marked in the plastic on both sides of the dbus connector. i need to work on searching for howtos online that are closer to my speed. maybe something that starts with:

step 1: remember to keep breathing before, during and after the activity.

jakethompson1 wrote on 2023-03-02, 15:43:

If this is for DOS CTTY COM1, be aware that DOS needs to see DSR set or otherwise it will give you abort/retry/fail forever.
If it's one of those modular cable ends it might be possible to open it up and modify it to connect DSR with CD or some other pin that is connected to DTR at the opposite end.

I think the same, but I also vaguely remember that MS-DOS 5/6 may be slightly different from PC-DOS 3.x and other DOSes.
For example, copying files with COPY FILENAME.EXT /B COM1 /B and COPY COM1 /B FILENAME.EXT /B does work with classic DOSes.
Or using COPY CON COM1.. On DOSes like FreeDOS, this may no-longer work properly, not sure.
Some DOSes seem to react to special characters, escape sequences etc.

Also, there's the AUX device, as an alias to COM1. It's a legacy from CP/M.
However, it can't be set by MODE for some reason. Maybe it's still useful for certain tasks.
If COM1 fails for whatever reason, I recommend trying the same with AUX, too.

PS: I do remember a great article/site about null-modem cables. ^^
A Null-Modem is a Null Modem, right?

Edit: Please don't worry about wiring when it comes to classic serial ports!
Unlike Centronics port, the RS-232 on PC has protective diodes built-in. Shorts don't kill it normally.

Edit: Also worth mentioning is the FiFo issue. The 8250 and 16450 UART have no working FiFo buffer.
On plain DOS, you can still reach high baud rates, maybe, but it's a risk. Speeds over 9600 or 19200 may cause data loss.
So on Windows 3.x or OS/2 or any other OS with multitasking, a 16550A (AD) or familiar is a must.
The modern FiFos also have a lower interrupt rate, which means that they don't slow down the CPU so much.
That's useful when using a serial mouse, for example. The 16550 series is pin compatible to the 16450 chip.
More information: Re: File transfer over serial to MSDOS5 8086 computer