[Elecraft] XG-3 - why RS-232

[amsctalx at comcast.net]

Topic by topic: 

>Many microprocessor controllers have...built-in serial ports 

Correct, and usually I2C, EtherNet, SPI and a host (pun intended) of communications peripherals. But even the beefy Motorola/FreeScale SCI (similar to a UART) ports need, in almost any implementation that I am familiar with, buffer circuitry - I hope no-one is routing the uC directly to a connector. 

USB needs a similar buffer circuit, known as a transceiver, which provides signal-level conversion and isolation. That's it...there is no UART in the classic sense. 

And RS-232 serial ports are rapidly become scarce on microcontollers...just go their websites and read the datasheets. 

>The low power line drivers consume less power, need fewer parts (no crystal), can be smaller than the USB UART and are likely to be >more reliable. 

Again, no-one really needs free-standing USB protocol units (chips) anymore, so the "crystal" comment is irrelevant. 

Aside from peripheral power supply capability, which is optional, the signal driver power demans are actually similar, the USB using a low-voltage differential-mode signal with moderate drive currents, and RS-232 are high-voltage (5-25V), single-ended signals with low drive currents. 

As for reliability...there are a thousand factors to be considered, and I don't see any data anywhere to back that up. 

>If you want USB... 

That's where we run into problems. "Adapters" are inherently less stable than well-implemented on-device peripherals. That's why people such as myself want native USB capability - conversion is not the same, despite assertions I have seen in this mail list and in other forums. 

I realize there will be a cost delta between RS-232 and USB, and there may not be a business case on a hobby product. I can live with that on some devices, such as the XG-3, but I am really beginning to struggle with RS-232 on transceivers and similar devices. It just doesn't make a lot of sense to me anymore, and that's coming from a person that has done embedded system design using both RS-232 and USB. 

I'm not knocking Elecraft, but I think that, based on today's PC market and the maturity of the alternatives, the era of RS-232 as the only choice should be drawing to a close. 



Mike Alexander - N8MSA 

amsctalx at comcast.net 

----- Original Message -----
From: "Joe Subich, W4TV" <lists at subich.com> 
To: elecraft at mailman.qth.net 
Sent: Saturday, May 12, 2012 10:42:37 AM 
Subject: Re: [Elecraft] XG-3 - why RS-232 


On 5/12/2012 9:17 AM, Pete Smith N4ZR wrote: 
> I would have thought that a USB port would have advantages so far as 
> space and weight, maybe even power consumption, in a battery powered 
> miniature device. 

Not so ... many microprocessor controllers contain one or two built-in 
serial ports. It is far cheaper to adapt those TTL (or 3.3V) ports to 
RS-232 "compatible" operation (typically a single line driver) than to 
install a USB UART and deal with drivers for a multitude of operating 
systems. 

The low power line drivers consume less power, need fewer parts (no 
crystal), can be smaller than the USB UART and are likely to be more 
reliable. 

If you *want* USB interface to the XG-3 pick up one of the USB "scanner 
programming cables" - typically USB to RS_232 compatible 3.5mm stereo 
plug. I believe Elecraft have such a cable as the KXUSB (standard for 
the KX3) ... I don't know if they list it separately as a spare. 

73, 

... Joe, W4TV 


On 5/12/2012 9:17 AM, Pete Smith N4ZR wrote: 
> Not exactly comparable to the XG-3, perhaps. I would have thought that 
> a USB port would have advantages so far as space and weight, maybe even 
> power consumption, in a battery powered miniature device. For those 
> using old computers, USB port cards for the PCI bus are absolutely dirt 
> cheap. 
> 
> 73, Pete N4ZR 
> The World Contest Station Database, updated daily at www.conteststations.com 
> The Reverse Beacon Network at http://reversebeacon.net, blog at reversebeacon.blogspot.com, 
> spots at telnet.reversebeacon.net, port 7000 and 
> arcluster.reversebeacon.net, port 7000 
> 
> 
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