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Old 7th Nov 2017, 12:08 am   #1
Skywave
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Question Diagnostic for COM port in a PC

Many years ago, in the days of MS-DOS and Windows 3.1, I used to use the software diagnostic 'Check_It' with a loop-back plug to test a COM port on a PC. Those days have now long gone of course, but there are still desktop PCs in use that still have a COM port (albeit only one these days). Here I have a Pentium 4 that is happily running Windows XP + SP3, but I have my doubts over the functionality of its COM port.
Does anyone know of a suitable S/W diagnostic + loop-back plug (with the appropriate wiring instructions) to test such a COM port?

All replies will be appreciated.

Al. / Nov. 6th.
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Old 7th Nov 2017, 12:35 am   #2
SiriusHardware
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Default Re: Diagnostic for COM port in a PC

For just a basic test, take a 9-way D plug of the appropriate sex and solder a wire or drop a solder blob between pins 2 and 3 on the plug. Put it into the COM port in question.

Assuming you're running Windows of a certain age, go to Programs / Accessories / Communications (or something along those lines) and run Hyperterminal.

In Hyperterminal, refuse the opportunity to 'make a new connection' and then go File / Properties / and in the 'connect to' tab, go down to 'connect using' and choose the COM port in question. Then hit the 'Configure' button.

In the window that appears, set the flow control to NONE and leave the baud rate, parity, stop bits etc set as found.

Hit OK and OK again to return to the main terminal screen. Go up to CALL on the menu bar, in the menu which opens up, hit CALL, you will be asked for a connection name again, this time supply one.

You may also be advised that you should install a modem, decline.

When the box closes and you return to the main terminal screen, anything you type on the keyboard should appear on the screen. If it does, the characters you are typing are going out of the COM port on the TX pin, back in on the RX pin and then being displayed on the screen.

You can verify this by removing the link plug and typing some more characters on the keyboard. They should no longer appear on the screen.

This simple method does not test the handshaking lines, which we turned off by turning off flow control. Whether this proves enough depends on what you actually suspect is wrong with the port.

On a healthy output on a COM port with no transmission active, there will normally be a standing MINUS voltage of somewhere between -5V and -12V.
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Old 7th Nov 2017, 10:05 am   #3
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Default Re: Diagnostic for COM port in a PC

Back in the day, we used to use an expensive bit of kit called a Processor And Peripherals Electronic Reactive Communications Link Interface Probe, or PAPERCLIP for short. Take a paperclip straighten it and bend it into a U. Shove one leg into pin 2 and the other into pin 3 and you have a simple loopback test as in the previous reply. No plug or soldering required.
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Old 7th Nov 2017, 10:20 am   #4
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Default Re: Diagnostic for COM port in a PC

Quote:
Originally Posted by SiriusHardware View Post
take a 9-way D plug of the appropriate sex and solder a wire or drop a solder blob between pins 2 and 3 on the plug.
Won't you also need to strap some handshaking lines?

My 9-pin serial loop-back plug (also dating back to the days of Checkit) has the following straps:
1-4-6-9
2-3 (as stated above for data)
7-8

No doubt, not all these are needed, but I would have thought that an RTS-CTS and/or DSR-DTR strap would be needed as a minimum. I would need to dig out the pinout diagram to remind myself which pins those are.
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Old 7th Nov 2017, 10:30 am   #5
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Default Re: Diagnostic for COM port in a PC

If you switch off 'hardware handshaking' in the communications software (Hyperterminal is horrible, I recommend and use TeraTerm), you won't need any of the handshaking lines to be connected.

I routinely use the PAPERCLIP method for testing serial ports, though the paperclip can be readily subsituted for a screwdriver blade, pair of tweezers, or more or less anything else lying around on the bench...

Chris
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Old 7th Nov 2017, 10:39 am   #6
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Default Re: Diagnostic for COM port in a PC

Ah, I missed the bit about setting flow control to "none".
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Old 7th Nov 2017, 2:20 pm   #7
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Default Re: Diagnostic for COM port in a PC

To use a PAPERCLIP easily on a PC, you'll need a F_F Gender Bender!

Or maybe you could get away with a solder bucket 9 D female connector.

(but if you've got one of those, a pukka soldered link would be easier anyway).
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Old 7th Nov 2017, 8:06 pm   #8
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Arrow Re: Diagnostic for COM port in a PC

Ref.: post #2 by SiriusHardware.

Thank you very much: it certainly helped. Although I've 'looked at' hyper-terminal which comes with various releases of Windows, I've never explored it. Your instructions were clear and, moreover, they worked. The only comment I will add is that although the routine (along with TXD and RXD) checks the RTS / CTS hand-shake lines, it doesn't do likewise for DSR / DTR, plus DCD and RI aren't featured. Useful, nevertheless: thank you.

Al.
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Old 7th Nov 2017, 8:28 pm   #9
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Default Re: Diagnostic for COM port in a PC

Quote:
Originally Posted by cmjones01 View Post
(Hyperterminal is horrible, I recommend and use TeraTerm),
I agree Hyperterminal is fairly horrible and doesn't lend itself nicely to any application which just wants to talk to something through the comms port, but it is the de facto 'factory fitted' terminal on Windows and everyone has it including me, so I chose to use that for the walkthrough.

I'm not sure that it is still present by default in Win8 through Win10, but I copied the .exe over from an earlier Windows. made a desktop shortcut to it and it actually works fine under both OSes, assuming there is a COM port for it to find. (It works OK with USB 'COM' devices as well).

Skywave, I know Hyperterminal doesn't allow you to check everything, at least by itself. I wasn't sure whether you just doubted whether the COM port was working at all (ie, blown TX output or RX input), which was one question easily settled using Hyperterminal and a wire link.

Last edited by SiriusHardware; 7th Nov 2017 at 8:38 pm.
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Old 7th Nov 2017, 8:53 pm   #10
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Default Re: Diagnostic for COM port in a PC

Termite is a very simple and powerful serial terminal, runs as an .exe without installing on windows. One of those free programs I felt donating to.
 
Old 8th Nov 2017, 2:38 pm   #11
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Arrow Re: Diagnostic for COM port in a PC

Quote:
Originally Posted by SiriusHardware View Post
Skywave, I know Hyperterminal doesn't allow you to check everything, at least by itself. I wasn't sure whether you just doubted whether the COM port was working at all (i.e. blown TX output or RX input), which was one question easily settled using Hyperterminal and a wire link.
The 'requirement ' arose after I had acquired yet another HP LaserJet 4 printer. First thing: check it. Then I discovered that I didn't have an available Centronics parallel data cable which I could devote to this printer. However, I recalled that many, many years ago I did have an HP LJ3 running from a serial port, so tried that - but without success. In the meantime, I ordered a new Centronics cable - which arrived today - and the printer works perfectly.
Although I knew that the 'requirement' would be easily and cheaply met by buying that cable, it also made me realise that I do not have the necessary kit to check the COM port on any PC here. But I have since discovered that indeed I do have such-like! However, a minor repair is required on same, (parts are on order), so I'll return later with the full story.

Al.
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Old 11th Nov 2017, 4:28 pm   #12
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Arrow Re: Diagnostic for COM port in a PC

... And here is the rest of the story.

I recalled that somewhere I do have my 'PC comms. test kit', left over from the days when I was a Field Service Engineer doing PC maintenance anywhere in the south of England. With that remark, I'm now going back at least 20 years and it took me sometime searching before I found it - in a cardboard box at the back of the attic!
Amongst other things, there is a cable test box (a re-housed 'Black Box' item); an MT25 'Interfaker' Mk.3; a suitable loop-back plug for same; a 9-way D-type 'straight through' cable; a 9-way to 25-way adapter cable, and a general purpose diagnostic disk for PC hardware (photos are below).

The 'Interfaker': for those who are not conversant with this item, in essence it is a 'break-out' box for testing, monitoring & configuring RS-232 / V.24 interfaces. (From now on I shall only use 'V.24' for brevity). As built, it is intended to sit between a DCE device (such as a V.24 modem) and a DTE device (such as a V.24 terminal). The Interfaker not only monitors and displays the status of many of the lines in the interface but also allows for a manual configuration of same by means of on/off switches, jumper cables and L.E.D.s. The Interfaker has two 'patch panels (25-W D-type) to facilitate such configurations. Thus, it can also be use to check for / configure DTE-DTE comms. It is powered by a 9-volt battery.

However, upon initial inspection and test, I soon discovered that many of the individual sockets in the 25-W D-types were making a loose fit with the jumper cables that are used with this device. So these were D-types were promptly replaced. (I say 'promptly' somewhat with tongue-in-cheek, since the Interfaker PCB has tracks on both sides with PTH and trying to get all 25 solder pins of the D-types correctly aligned to the holes in the PCB was not particularly easy!). But patience won the day.

The rest is quite straight-forward: hook-up the hardware with the appropriate loop-back plug, load the software, select the appropriate test - and run it. The Interfaker LEDs either flash or are static (according to which lines are being tested) and the results displayed on the screen - as per the photo below. In this case, the COM port (on this PC) was given the 'all-clear'
The one point that is worth mentioning is that this diag. S/W cannot be run if any O/S is running. A fresh re-boot is required, booting from the FDD on which the diag. is located.

Finally, an 'aside'.
The HP printer referred to is now happily installed and running, having obtained a 25-W 'D' to Centronics 36-W cable and using the printer's parallel I/F from the PC's LPT1 port.

Al.
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Last edited by Skywave; 11th Nov 2017 at 4:32 pm. Reason: Add the 'aside'.
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Old 15th Nov 2017, 1:41 pm   #13
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Arrow Re: Diagnostic for COM port in a PC

It's quite clear to me that there is no more interest in this thread: it has run its course. So perhaps it's time to close this thread. My thanks to everyone who has contributed to it.

Al. / Nov. 15th.
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Old 15th Nov 2017, 1:46 pm   #14
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Default Re: Diagnostic for COM port in a PC

Threads are closed automatically 180 days after they're last posted in.

https://www.vintage-radio.net/forum/...d.php?t=140775

The exception is Sales and Wants threads which are closed manually once a deal has been done.
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Old 15th Nov 2017, 2:17 pm   #15
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Default Re: Diagnostic for COM port in a PC

In times-past pretty much everyone who dealt with datacomms had a set of 'loopback' plugs for quick-and-dirty interface testing.

The standard RS232/V24 25-pin-D one had the following links:

2-3
4-5
6-8-20
15-17-24

There was a similar version for 9-pin but it omitted the clocks [15/17/24 on the 25-way] so was only suitable for asynchronous data.

I forget what the wiring was for the more-common V.35 MRAC-connector version used to distribute 56/64Kbit-sec data/voice circuits (it's a long time since I needed to talk to Ma Bell on private transatlantic links)
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Old 15th Nov 2017, 9:27 pm   #16
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Default Re: Diagnostic for COM port in a PC

As a vintage computer enthusiast I have many machines with RS232 ports, and a reasonable collection of test equipment including line monitors, breakout boxes, etc.

I find one of the most useful testers is a little adapter with a DB25 plug on one and and a DB25 socket on the other, with 7 bicolour LEDs and series resistors monitoring TxD, RxD, RTS, CTS, DSR, DTR and DCD. It will indicate if a port is getting power at all, whether it's wired as a DTE (Terminal) or DCE (Modem) by which LEDs light, which handshake outputs are driven and so on. It's saved me many hours... At one time these were easy to get, now of course they are harder to find but do turn up on Ebay.

Pet hate : people who talk about DB9 connectors. There is no such thing, they normally mean DE9.
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Old 15th Nov 2017, 10:32 pm   #17
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Default Re: Diagnostic for COM port in a PC

For quick checks and for casual Rx and Tx sniffery I still use an old Pocket PC for RS-232 based stuff.

It's about the size of a mobile phone and I wrote a few RS232 diagnostic apps for it in eVB. It really is a neat tool and hardly takes up any space and the app is reliable and doesn't have all the setup complications of the various commercial terminal programs. Just connect to the COM port and go! The alternative is a laptop or other PC but these take up a lot of space. My (HP Ipaq) Pocket PC was quite limited in what it could do but for stuff like this it was hard to beat because it was so small and handy.
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Old 16th Nov 2017, 9:41 pm   #18
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Default Re: Diagnostic for COM port in a PC

Jeremy's post prompted me to experiment with an old tablet and a USB to serial converter to give a portable solution.
I found an Android App USB Serial Terminal Lite that looks promising.
Received data can be displayed as ASCII, Hex or Binary, Transmitted data is ASCII by default but Decimal, Hex or Bin can be sent by using a prefix (@, # or &).

With a fully implemented USB- RS232, DB9 unit (FT232 based), DTR and RTS outputs can be set as required and CTS, DSR, CD & RI inputs monitored.

A cheap USB-TTL converter (PL2303 based) with RX & TX only also worked.

Jim
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Old 17th Nov 2017, 12:23 am   #19
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Question Re: Diagnostic for COM port in a PC

In my post #12, last photo., the 25-way 'D' type pin 11 is titled 'NTCL'.
Anyone know what 'NTCL' is an abbreviation for, please?

Al.
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Old 17th Nov 2017, 2:56 pm   #20
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Default Re: Diagnostic for COM port in a PC

NTCL is a bit of a mystery. It's shown against pin 5 of the 9-pin connector, which is definitely the ground pin, but against pin 11 of the 25-pin connector. Most 25-pin serial pinouts show pin 11 as unassigned, but some synchronous serial interfaces use it as a clock pin. My best guess at an expansion for NTCL would therefore be 'Not Transmit CLock', assuming an active-low clock. However, PC serial ports don't usually support synchronous operation so I'm not convinced.

Chris
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