■ SCREEN SHOT 5. The next expected
seq number is actually not part of the
TCP protocol. The sniffer is assisting the
user here by informing him or her that
24 bytes of data has been passed to the
laptop from the Frame Thrower II. If you
include the null (0x00) terminating
character, the Telnet banner data is 24
bytes in length.
sequence (0D 0A).
At this point, the TCP application
can step in. In the minimal TCP/IP
stack code I have provided via the
Nuts & Volts website ( www.nuts
volts.com), the TCP application is a
simple echo routine. Each character
entered in the Telnet session at the
laptop is received by the Frame
Thrower II and echoed back to the
laptop.
THE FRAME THROWER II
HARDWARE
Much of what has to be said
about the Frame Thrower II hardware
is stated schematically by Schematic
1. The EDTP Frame Thrower II is an
integrated 10Base-T Ethernet module. The Frame Thrower II is based on
the Microchip ENC28J60 Ethernet
Engine IC, which interfaces to just
about any host microcontroller that
supports a standard SPI interface. To
allow for easy experimentation and
application development, all of the
PIC18F8722 microcontroller pins are
brought out to standard .100-inch
center holes.
The Frame Thrower II comes
equipped with three onboard status
LEDs, which can be illuminated or
extinguished under program control.
A 10 MHz crystal is the seed for the
40 MHz PLL clock that drives the
PIC18F8722 microcontroller. The
ENC28J60 requires a 25 MHz crystal
for its internal clock and, as you can
see in Photo 1, the Frame Thrower
II circuitry includes the 25 MHz clock
source.
No communications project is
complete without a fully functional
serial port and a Sipex SP233 forms
the heart of the Frame Thrower II’s
regulation RS232 interface. To keep
things as simple as possible on the
Ethernet side of the equation, the
Ethernet physical interface is built
around a consolidated magnetics/
RJ- 45 MagJack assembly from Bel
Stewart.
To run at full speed, the
PIC18F8722 must be powered from a
+ 5.0 VDC supply. However, the
ENC28J60 is strictly a + 3. 3 VDC part.
To accommodate operation of both
the PIC18F8722 and the ENC28J60,
the Frame Thrower II incorporates a
dual + 5.0 VDC/+ 3. 3 VDC regulated
power supply subsystem.
If your application is time
sensitive, an optional real-time clock
can be implemented by adding a
32.768 kHz clock crystal and a couple
of 20 pF capacitors to the Frame
Thrower II printed circuit board. A
25LC256 EEPROM has been included
to allow the network programmer to
store web pages. However, the
25LC256 can be used for anything the
network programmer (that’s you, by
the way) desires.
I would normally provide an
ExpressPCB layout for the Frame
■ PHOTO 1. The EDTP Electronics, Inc.,
Frame Thrower II is a complete
embedded Ethernet node. Not only
does this little bugger run our simple
TCP/IP code, it can also run the more
sophisticated Microchip TCP/IP stack.
88
September 2006