[aprssig] Alinco DR-135T - EJ41U internal modem

Curt, WE7U archer at eskimo.com
Tue Nov 1 12:13:14 CST 2005

On Tue, 1 Nov 2005, John Hansen wrote:

> It depends on just how poorly designed it is.  Suppose that it never
> envisioned getting two packets in a row.   With continuous reception,
> you have to have enough buffer for two frames, because you have to hold
> on to the first one until you can check the CRC and decide what to do
> with it.

Actually you can process the CRC while the bits/bytes are coming in,
then just do a quick compare when the CRC actually comes in, so this
isn't much of a problem.  I've done byte-wise CRC on a 6803 at
0.79MHz in assembly language no less.

> However, while you are checking the CRC and sending it out to
> the terminal, you've got to be receiving the next frame.  So regardless
> of the terminal rate, you need room for two frames.  If you designed it
> only to receive one frame at a time (that is, you could process the
> first frame before you had to worry about getting the next frame) and if
> it was a circular buffer that didn't have any protection against the
> second frame overwriting the first, really bad things could happen.  But
> it would have to be an incredibly poor design.

As long as you had the serial-port speed set above your radio speed
even the room-for-one-buffer problem shouldn't be an issue on
receive as you'd dump the packet out faster than the incoming packet
could overwrite the circular buffer.  Even at 1200 baud you
shouldn't have a problem unless you're not stopping to calculate the
entire CRC AFTER the frame is fully received.

> > Or is the buffer problem only when sending?
> This is more likely the case.  You can't send until the channel is clear
> so you have to have some storage capacity.

If you have multiple programs talking to the KISS TNC (easy to do on
Linux) then this could be a BIG problem.  Often the TCP/IP guys run
1024-byte packets too.  You really need some way to drop packets
coming from the computer if you can't handle what it's sending.

Curt, WE7U.   APRS Client Comparisons: http://www.eskimo.com/~archer
"Lotto:    A tax on people who are bad at math." -- unknown
"Windows:  Microsoft's tax on computer illiterates." -- WE7U
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