I assembled my current computer a few months ago. The MB has headers for a parallel port and a serial port, so I installed a plate at the back of the box with a DB25 and a DB9 connector, and plugged the ribbon cables onto the headers. But, I never tried to use either one until yesterday when I plugged in a newly constructed serial cable for PICAXE programming; I tried it and it didn't work, so I assumed the cable was wrong. However, the cable is good (as proven on another computer) and the port is bad as proven with another cable. Device manager indicates the com port is functioning properly, but still no go at the DB9. I rechecked the ribbon cable was inserted correctly, and it is, but now I am stuck.

It's probably something stupid I have or haven't done. Any suggestions on how to proceed?

Thanks.

 

Check the bios if it's enabled.

 

Check the bios if it's enabled.

I forgot to mention that I did check the BIOS. It was set to a particular interrupt and I changed it to auto select. I'll change it back.

 

Okay, I just rebooted to BIOS, and changed the com port back to the fixed IRQ4, and restarted, so it's back to factory default. Now, I am wondering if Device Manager would show it at all if it had been disabled. I think I'll disable it and see.

ETA: I disabled it in BIOS and it disappeared from Device Manager. I'll go back and re-enable it.

 

If you meter the tx pin to the signal ground pin do you have voltage?

 

If you meter the tx pin to the signal ground pin do you have voltage?

Negative probe on signal ground. Tx measures -.072V. Rx measures -11.17V. That seems wrong.

 

Negative probe on signal ground. Tx measures -.072V. Rx measures -11.17V. That seems wrong.

Is it a DTE or DCE header/cable?

 

I am not sure if I am answering your question or not, but the connector in the PC is a DB9M and the connector on the cable is a DB9F. If that isn't what you are asking, try "in other words."

Thanks.

I pulled the plate out of the PC. From the plug side and the five (male) pins on the top, they are labeled from left to right 1 2 3 4 5 and bottom row is labeled left to right 6 7 8 9. There is also a D molded into the plastic.

ETA: The device that I am connecting to the com port only uses three wires: TxD, RxD, and signal ground.

 

Okay, I think I have found the problem. The connector on the end of the ribbon cable from the DB9 fits the header on the motherboard, but the connections are not in the proper order. The accessory plate that I have wasn't made for the motherboard, and apparently the wires are not in the correct order to bring the leads out properly.

I thought that was standard across all MBs and cables, but I guess not.

 

Which OS are you running?
Do you need extra device drivers?
Is the cable properly installed on the motherboard?

 

Which OS are you running?
Do you need extra device drivers?
Is the cable properly installed on the motherboard?

Win 7, drivers are installed, cable is apparently not correct for the MB I am using. See my previous post.

 

Which motherboard do you have?
Any documentation on the pinout on the motherboard?

 

Okay, I think I have found the problem. The connector on the end of the ribbon cable from the DB9 fits the header on the motherboard, but the connections are not in the proper order. The accessory plate that I have wasn't made for the motherboard, and apparently the wires are not in the correct order to bring the leads out properly.

I thought that was standard across all MBs and cables, but I guess not.

Time for a little 'solder magic" to fix the cable.

"standard across all MBs"

 

Which motherboard do you have?
Any documentation on the pinout on the motherboard?

GA-78LMT-USB3

The manual includes the motherboard pinout, and the cable I have does not bring the correct leads out to the correct DB9 pins. I am going to rewire the connector to correct the problems. Thanks.

 

Time for a little 'solder magic" to fix the cable.

"standard across all MBs"

Famous last words, huh?

I am heating the iron.

 

16 wire ends stripped, 8 solder joints made, 8 joints insulated = problem resolved. (One wire was correct.)

I actually think the root cause is a mistake on the MB PCB layout. I am sure that Gigabyte knows of it, but they decided that it would not be found by most users since most users don't need a com port. Lucky me!

 

Another successful demonstration of