RS232 serial modem word

[Mark Elkin]

Hi everyone,

I’m wondering if someone could help a beginner, who is thoroughly confused:

I am attempting to connect a real retro serial modem to my RC2014, using this board connected to the portB serial port:

I created my own db9 to db25 pin cable using pinouts I found online for a straight through serial cable, pin 2 of db9 to pin 3 of db25 etc.

I am using stern, patched to communicate via port b.

When I tried this set, I received nothing from the modem.

I then purchased this cable:

With this cable I receive the read message from the modem but it does not respond or receive anything I type.

I used a meter to test the purchased cable and mapped its pinout and found pin 2 of db9 was connected to pin20 of db25. ChatGPT identifies my purchased cable as a null modem cross over cable.

I am utterly confused:

Why does my straight through cable to work at all?

Why does a cable, labeled as a serial modem cable appear to be a null modem cable and why does it at least work in one direction?

Any advice on the correct pinouts, cable to make, would be greatly appreciated.

In summary:

This does not work:

1-8

2-3

3-2

4-20

5-7

6-6

7-4

8-5

9-22

This receives from modem but does not send to modem:

1-7

2-20

3-2

4-3

5-8

6-22

7-5

8-4

9-6

Thanking you,

Mark

[Mark Elkin]

[Mats Gülich]

Hi Mark!
As an "old" modem guy, may I be able to help you. I am though a bit confused about the pins you describe.
You have a serial 9-pin D-Sub connector and a USB. 
Is your question how to connect a 9-pin D-sub to a 25-pin D-Sub?

A straight cable look like this

9-Pin   25-Pin   Signal

1 - 8                    Carrier Detect (CD)

2 - 2                    Received Data (RX)

3 - 3                    Transmit Data (TX)

4 - 20                  Data Terminal Ready (DTR

5 - 7                    Common Ground (GND)

6 - 6                    Data Set Ready (DSR)

7 - 4                    Request To Send (RTS)
8 - 5                    Clear To Send (CTS)
8 - 22                  Ring Indicator (RI)       RI may only be used if a telephone line connected modem shall answer.

I can provide you with a crossed cable also between 9-Pin D-Sub to 25-Pin D-sub. Just be sure that 5-7 is connected properly, as it is the signal ground.. Please avoid to connect protective ground to more than one end, it may cause problems with creeping currents.

Friendly regards
Mats

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[Mark Elkin]

Hi Mats,

Thank you so much for your reply. I am not using USB, just db9 to db25 as you describe. 

I wired up the cable as per your pinout above and now my cable behaves like the one I purchased - in other words, when I turn on the modem, I get the welcome message, but nothing I type is echoed back and typing AT<enter> does not return anything.

It still seems that I can receive but not transit. 

I'm going to see if anything else is interfering with portB

Mark

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[Mats Gülich]

OK! I understand. Can it be so that your modem is not in Echo-mode?
If you shorten the cable between PIN 2 and 3 in one end should what you are writing be echoed back. See that your port is not using the Hardware handshake when you do that test.

Mats

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[Mats Gülich]

PS

As I try to recall it in my memory is the "ATEx" command for echo or not echo. Maybe ATE1 for echo and ATE0 for not echo, or the opposite. I tried to google it but didn't find the ATE command in the list I was looking in. TRY! :-)

DS

[Mark Elkin]

Update!

I just wanted to thank everyone for their responses and report that I got my modem working!

It turns out the board that converted TTL to RS232 was not bidirectional, I purchased another one and everything is working!

Now to find some old BBS sites.

Mark

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[Tim Radde]

What board did you finally get that fixed it?

[Tom Storey]

Bear in mind for the future some differences between a PC and modem...

A PC is considered DTE (Data Terminal Equipment) while a modem is DCE (Data Communications Equipment). The signal names on the DCE end match the corresponding signals on the DTE end.

So when wiring up DTE to DCE you need to match signal names: TX to TX, RX to RX etc

Only when wiring DTE to DTE (perhaps also DCE to DCE) do you cross signal names (null modem): TX to RX etc

Our hobby projects involving serial ports would be considered DTE for the purpose of naming signals.

USB to serial adaptors should also be almost universally considered DTE, unless otherwise specified.

[Mark Elkin]

This one did NOT work:

This one DID work:

The only subtle difference is that the Amazon listing specifically stated that the red one is bidirectional and if you look closely at the PCB, it has arrows showing the direction.

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[Tom Storey]

The blue one also has arrows to indicate the direction, they are beside the LEDs it looks like.

It seems odd that one would be specifically unidirectional. Perhaps you just had a dud unit?

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[Mark Elkin]

I thought it odd too, but I tried two separate blue ones and they did not work. I’m not sure why.

Sent from my iPhone

On May 16, 2025, at 5:19 AM, Tom Storey <t...@snnap.net> wrote:



The blue one also has arrows to indicate the direction, they are beside the LEDs it looks like.

It seems odd that one would be specifically unidirectional. Perhaps you just had a dud unit?

On Fri, 16 May 2025 at 10:35, Mark Elkin <mselkin....@gmail.com> wrote:

This one did NOT work:

<image.png>

This one DID work:

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[Andrew Carr]

Hey all,

A few months ago I was having the same issues with a TTL/RS232 adapter like the ones in the images.

It turned out to be 2 issues at the end. One is that I needed to provide power via the TTL interface (that's on me, totally thought the RS232 would be providing the needed voltage, for whatever reason...) but the big issue?

Counterfeit/Dead MAX232 ICs. From suppliers both on Amazon and eBay.

When I plugged the 'dead' adapters in and tried to use it, the chip heated up enough to give me a decent 1st degree burn in 5 seconds when I was poking at it.

In the end, I ended up buying one from mouser or digikey, since at least then I could talk to someone on the phone if it was a bum unit...

Andrew

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[Mark Elkin]

Yes, as a beginner I've run into that a lot. I purchase parts off Amazon or eBay, based on an online tutorial and 50% of the time they do not work, then, after trying multiple versions of the exact same board I eventually find one that does. The quality from Amazon and eBay is disgusting.

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[Mats Gülich]

You mix up expressions about the equipment with the signals. DCE and DTE. It has nothing with the signals in the RS232 interface to do. DCE and DTE just tell you h´which is the Tremainal (DTE) and the "modem" DCE.
DTS is a signal to connect the modem to the "line" DTR is an answer that the line (relay) is connected to the line. :-)

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[Tim Radde]

I rarely have had problems with stuff I purchase on Amazon. Then again I am not usually buying this type of thing there.  These days you never know what's made in China and may be good or fake.  I rarely buy things on eBay any more.

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[Spencer Owen]

On Fri, 16 May 2025, 14:57 Andrew Carr, <tan...@gmail.com> wrote:

When I plugged the 'dead' adapters in and tried to use it, the chip heated up enough to give me a decent 1st degree burn in 5 seconds when I was poking at it.

Oooh! I got my fingers burnt on a MAX232 last year for Retro Challenge and I still have the (mental) scars. 150°C isn't a chip temperature I'm happy touching! 

I wrote up my experience on the RC2014 blog, along with some suggestions that had been made but didn't work for me.

Https://rc2014.co.uk/3201/retro-challenge-2024-10-getting-my-fingers-burned/

Spencer