Really new to the circuit building/electronics area so please bear any elementary questions and mistakes!

I've searched the web and have found schematics for other uses but can't seem to adjust them to my purpose, at least without causing some nasty stuff which I can't afford!

I have a device that can be connected via RS-232 and USB to a PC so as to send/receive data.
The operation is pretty simple for the Serial as it employs Rx, Tx and Ground, while the USB uses all four connections. In reality the Serial connection is only used for sending data to the PC (i.e. no Rx on the device so I can skip this).

As the device is physically located in a cramped space without easy visual reference, I want to place an "intermediate" panel with plugs for the Serial and USB cables near the PC. In order to at least see if the data is flowing, I want to build a (or two) circuit(s) and have two LEDs apiece to indicate data flow from the USB/Serial to the PC and vice versa.
As I mentioned I've found schematics mostly for USB but I'm a bit confused with the resistors required plus I'm not sure if the data signals will get degraded (which I don't want). Things get more complicated with the Serial signals as the actual data are in the 0 to -13V area so if I understand correctly I'll have to use a diode to get the LED working.
I've also found many schematics that employ microcontrollers which I really don't need for this project. So basically just a "pass through" circuit for data flow indication without degrading the signals before they reach the PC.

Any help would be greatly appreciated!

 

You could have LEDs monitoring the state of the TXD and RXD signals. Normally these signals are between -10V and + 10V (approximately). When there is no data transmission both these signals will be negative (the mark condition).

A simple way is to have a low-current LED with a series resistor between TXD and COMMON and RXD and COMMON. If the current is less than a milliamp you will probably not degrade the signals too much. So at 10V with a red LED you need a resistor of about 8k2. With a green LED - that has a higher forward voltage make the resistor a bit less - say 7k5. Connect so that the LED lights when the signal is positive. Because the signals go negative, connect a diode (1N4148) in reverse parallel with the actual LED. Make sure you use low-current LEDs that can work with 1mA or less.

Of course the data rate will be too fast for you to see much - but you will see when data is passing through.

 

Many thanks Marley!
I'm attaching a crude circuit plan based on what I had "mashed" from other designs and what you wrote. As I said I'm really green so even basic stuff may well elude me as I'm sure is true in the attached image. I had already included the 1N4148 which from other designs I understand that is required for the Logical 1 (i.e. negative voltage). However, I'm not sure about the "reverse parallel"
For the data transmitted from the PC to the device (right to left in the image), I've included a yellow LED which I think has the same fV with the greeen hence the same resistor (7k5).

The blinking effect is not an issue, the main goal is to see that there is a good data flow so the LED blinking to show "data rate" would be a luxury or something for version 2 of this circuit as soon as I've understood the basics.

I'm pretty sure though that I've messed up the connections! Any guidance would be most appreciated!

 

Your diagram is close but you are shorting the serial lines to ground at each end!

By inverse parallel I meant like the diagram attached. I have placed the LEDs so that when there is no data and the serial lines are negative, the LEDs will not be lit.

You will need LEDs that can work with low current. Red and green are probably best. Red because they are the most efficient and green because our eyes are more sensitive to green than any other colour. Not sure about yellow.

Edit: whoops! first drawing diodes wrong way round - corrected now!

 

Many thanks Marley this is great! My approach didn't seem right from the start.
Good tip for LED color too, thanks! To be honest, the main issue is the Equipment->PC route rather than the opposite So I can use the one LED only but it will be helpful to have both.

So the respective USB circuit would be the same but with one set of diodes/LED/resistor? It's a bit more confusing as there are no two dedicated pathways as in the Serial but the differential pair for data. Additionally, would a two-color LED work? I'll need to check on consumption as I want to avoid signal degradation as with the Serial.

Once again, many thanks for your help.

 

No. The USB circuit is completely different. Very hard to monitor this in any meaningful way as there is other data and clock signals on the USB at all times.

 

Yeah I feared as much. Ready solutions I have seen ("naked") usually have pretty loaded PCBs.
Just to bug you a little further though, the "simple" rhythmic flashing you usually find in USB sticks? So basically just regular flashing when data is saved on the stick. I have found a simple schematic for a USB Tester (aptly named Simple USB Tester!) which has three LEDs for power, reverse polarity and data flow. However as it is a tester the data LED is connected between 5V and D+ (as attached) so I'm not sure if it would work on a real circuit.

 

Detail: any particular flavor of resistor, i.e. Metal film, carbon etc? And also Watts? Same for the 1N4148 as to Load Current and Max Current?

 

On the USB side you really can't monitor those signals so slimply, but your PC can see the device in anything like device manager.