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MFJ-1278B MULTI-MODE HARDWARE
Inverter U10e buffers the radio port "16x" baud-rate signal in case it is routed, via modem
disconnect J4 pins 11 and 12, to an external modem. If this buffer were not included,
reflections from the distant termination might cause counter U1 to generate count errors.
Counter U4b provides a properly scaled clock for the transmit NRZ to NRZI encoder (see
Serial Interface, below).
CPU Complex
EPROM U23 provides system ROM for program storage. Selector U12a acts as a ROM
decoder, mapping the ROM into the CPU's memory address space beginning at address 0.
Static RAM provides system RAM for temporary scratch-pad storage, message buffers, etc.
Also, because the RAM is backed up by a battery and will not lose its contents when the main
power is removed, it is used to provide semi-permanent storage of user-supplied information,
such as your callsign. Selector U12b acts as a RAM address decoder, with RAM starting at
address 8000 hex.
The sections of CMOS switch U13 are used to insure that the RAM is not selected when main
power is removed. This ensures that the contents of the RAM are not accidentally scrambled
as the CPU loses power; it also ensures that the RAM is in the "power-down" state for
minimum battery power consumption.
Serial Interface
Serial Input/Output, or SIO device U21, provides two channels of serial I/O.
The B SIO channel is used for the computer or terminal interface. Operational amplifier
sections U3a, U3b, and U3d act as RS-232C drivers while Schmitt trigger inverters U9a and
U9b act as RS-232C receivers. These circuits consume less power than conventional RS-
232C drivers and receivers.
The A SIO channel is used for the radio/modem interface and is normally operated as a full
duplex HDLC channel for compatibility with the AX.25 protocol specification. Latch U5
and ROM U6 provide a "state machine" for recovering the clock from the received NRZI
data. The state machine also converts NRZI data to NRZ for the SIO. Inverter U9c and flip-
flop U11a provide NRZ to NRZI conversion for the transmit side of the radio channel. This
conversion between NRZ formatted data and NRZI formatted data is necessary because the
AX.25 protocol specification requires NRZI operation while the SIO is only capable of NRZ.
Jumper JMP11 may be used to bypass the NRZ and NRZI conversion for use with external
modems, if required.