MFJ-8621 Data Radio
4
As a "packet-only" radio, the MFJ-8621 is specially designed to transmit and receive all
data rates through 9600 baud. However, to accomplish this, the MFJ-8621 must
deliver
unsquelched AFSK or FSK data to the TNC. The MFJ-8621 does not provide
squelched AFSK because conventional squelch circuits are far too slow to be useful
above 1200 baud. In order to process unsquelched data from the MFJ-8621 (or any
high speed data radio), your TNC must have a data carrier detect (DCD) circuit built in.
All MFJ model TNCs -- starting with the MFJ-1270B-- and virtually any other TNC that
operates over 1200 baud should have one as well (PK-96, DPK-9600 for example).
Unfortunately, many TNCs, like the PK-232 and even some low cost mini-TNCs, lack
DCD because they are made exclusively for use with conventional FM voice radios to
run at 1200 baud. If your TNC does not have DCD, it cannot distinguish between
limiter noise (the strong rushing noise FM radios produce with no signal present) and
data signals. Since the TNC sees the limiter noise as incoming signal, it will not allow
the TNC to transmit due to the TNC's data collision protection function. Consequently,
the radio will appear incapable of transmitting! Again, your TNC
must have DCD to
work with the MFJ-8621.
If your TNC
does have a DCD, make sure the function is enabled in your particular unit.
Read your TNC documentation carefully to confirm what type of carrier detection--if
any--exists in your unit. If you
do have DCD, activate it.
Important Note:
Some TNC's have software DCD circuits that may also require internal adjustments
(check your TNC manual and follow activation instructions carefully). Other TNCs may
not have a DCD at all or have a DCD indicator that is really nothing more than a mis-
labeled "signal-present" indicator. Separate "universal" add-on DCD circuits are
available for TNCs that lack a DCD. PacComm offers add-on DCD circuits for many
applications and has information about choosing the right DCD to fit your needs.
Refer to the appendix for notes on specific TNCs.
CONNECTING TO YOUR MFJ-8621:
Power Supply:
The MFJ-8621 will operate from any well-filtered DC power source delivering 13.8
Volts at 1 Amp. The MFJ-4110 wall-adapter supply is a light-weight voltage-regulated
unit especially suited for powering the MFJ-8621. "CB" type supplies or battery packs
capable of 1-A output will also work (anticipate lower RF output when using a 12-volt
source). The MJF-8621 accepts a standard 5.5mm x 2.1mm coaxial-type power plug
(Radio Shack 274-1569). Connect the (+) or red lead to the center pin, and connect
the (-) or black lead to the outer sleeve. Use a 2 amp fast-blow fuse with any power
supply other than the MFJ-4110. Your MFJ-8621 features "crowbar" protection to
prevent damage from incorrect power supply polarity. If power leads are accidentally
reversed, the crowbar-diode conducts and immediately opens the radio's line fuse,
cutting off power (for line-fuse replacement instructions, see the Appendix).
