MarkO

Hi all...

I'm working on some software which communicates with a plasma screen via the serial COM port. However, my development machine is a laptop which doesn't actually have a serial port, and I don't have a plasma screen on-site with which to test. This is, to say the least, making the development process more complicated.

Anyway, the plasma screen issue isn't particularly important, as long as I open a connection to a COM port (which doesn't necessarily have to be connected to anything) and send data.

So, I need some software which can emulate a Serial port on my PC. So far I've found this:

http://www.virtualperipherals.com/vsp.htm

which is superb, and does exactly what I need. I can create a COM2 serial port, and send it data as if it's a real port.

However, it's only a 2-hour demo, unless you've got a serial number - which I don't have, and can't afford.

So, can anyone help me out - obviously by recommending an alternative bit of virual serial port software which is freeware (or even shareware)?

IWatkins

MarkO,

When you say you don't have a serial port, I'm assuming it is a newer laptop with USB only ports ?

If so, maybe try a USB - 2- Serial cable. Now, I'm not sure if they install as a "pretend" COM port or not though but might be worth a try.

There was a lot of discussion about these cables a while back. Maybe someone who has one could help out ?

There again, I'm not sure of the cost, so maybe a software only solution might be cheaper.

Cheers

Ian

MarkO

Thanks Ian, good suggestion. But since I don't actually have a plasma screen to plug into the serial port, buying a USB->Serial cable wouldn't help much, other than providing a COM port to test with.

However, USB/Serial cables are pretty expensive, and I really want a free solution (the work I'm doing is unpaid as yet).

The virtual serial port software I mentioned above seems to do the trick though, and I can actually use it quite a bit; the virtual port works for 2 hours, after which you need to reboot the PC to re-enable it. The evaluation period is a total of 30 days. So all-in-all I should get a month of testing before it's completely disabled, which should be plenty.

midget1500

i'd love to know how these USB->serial cables work, i.e. if u write software that actually deals with the com port (serial) at a hardware level then how the heck will these cables help? i take it there is some software emulation etc, but just can't figure it out in my head!

MarkO

Midget, the software doesn't actually see the serial port itself, it merely sees an interface to that port, through the driver. The driver is a 'black box' which takes data sent to/from it (by the software) and converts this into actual signals on the hardware pins themselves. So when your software writes data to the COM port, what it's actually doing is sending data into the driver interface. What actually happens inside the driver is irrelevant to the software using the 'port'.

This is how the HAL (hardware abstraction layer) on most modern OSes works. Devices have drivers which present a common, pre-defined interface to th software which will use them, so that the actual implementation of a device is irrelevant to the software using it.

So all a device has to do is to present itself (via the driver) as a COM port, and that's what the software will see, whether it's a 'real' serial port, or something else. Similarly, there's no reason why a serial port couldn't have a driver implemented which makes that port present itself to the system as, say, a screen or disk drive. It's all about how the software interprets the interface to the driver.

The best example is the one I'm using, above - a virtual serial port. My laptop doesn't actually have a physical serial port at all, but by installing a virtual serial port driver, a new port becomes available to the system. The fact that the port is only 'one-sided' is irrelevant - I can still send/receive data on that port as if it was a normal serial port, the only difference being that the data doesn't actually go down any physical metal pins, it just gets processed by some code within the driver.

Hope that makes things clearer... (probably not ).

midget1500

mark - yeah, i get that

but, i don't get how a cable can be plugged into a usb and link to a serial device - i.e. the computer will be driving a USB port and the device on the other end a serial port.

MarkO

Ah, well, that's simple. A little bit of circuitry in the USB/Serial cable does some signal processing to convert the pin inputs from the Serial end of the cable into a signal in the USB end. It's not just a 'straight-through' cable, hence the reason they cost upwards of thirty quid, instead of the usual fiver that would get you a null-modem cable.

The device driver for the USB Serial port then does some translation to make it appear like a Serial/COM device to the software.