On 5/19/2025 9:29 AM, Michael Mandell via groups.io wrote:
<snip>
With a normal USB cable, serial transmissions are broken into
packets, so your radio will transmit D-Rats serial data in chunks. It
goes on and off in xmit to get the whole thing out.
It should not be doing that unless somethings is wrong. Yes the USB breaks up the serial data into chunks, but it can fit an entire d-rats packet into one chunk.
What USB is doing is delaying sending any characters until its buffer reaches fixed mount, or there is a very small time that no additional characters are added to the buffer.
The radio has an XON/XOFF handshaking to slow D-Rats down to the speed that the radio can accept.
With the RT Systems cable and the radio data port, serial is a
continuous stream and it goes out in one shot.
I called Icom on this and got a support guy in Texas who explained
the whole thing to me. Both work, but the orange data cable (Icom
has one, too, but it costs way more) is a better solution.
I don't think you fully understand how that works.
D-rats transfers data to and from a USB serial port at a rate of about 30 Megabytes per second. In addition that transfer from the USB serial is done with what is known as a DMA transfer. Which means that it does hardly stresses your CPU utilization at all.
That means only one interrupt per transfer. Interrupts are costly for a PC to process, because it has to save context, process the interrupt and then restore it. The computer does this every time you do I/O except for screen writes. Most devices on your computer use DMA transfer.
On a "standard" older PC that still has a serial port, it has a very small buffer and must interrupt the PC once it has either filled the buffer or emptied the buffer. So at best, it is processing one of those interrupts on every two characters transferred. D-Rats sends/receives around 200 bytes per transfer, so that is at least 100 interrupts per transfer.
Now most people don't load up their PCs to the point where that makes a difference.
But your PC can handle at least 20 times the number of USB ports running flat out with data than the maximum number of "standard" serial ports it can support. And it was hard to even add enough "standard" serial ports to cause a strain on the last models of PCs that had them available.
Yes, USB delivers the first character in a bunch of characters slightly slower than the serial port. If you get a good enough analyzer you can see that easily. However your computer is working much harder to deliver that data on using the older serial port.
So use what ever cable you want to use.
73,
-John
