MFJ-1270C/1274C MANUAL HARDWARE
Page 209
The modem power sources and ground are isolated from digital logic switching
noise by inductor L2.
RS-232C Handshaking Protocol
The CTS, DSR and DTR lines of the RS-232C port (J1) are used for hardware
"handshaking" protocol to control the flow of data between the terminal (DTE)
and the MFJ-TNC (DCE).
The MFJ-TNC always asserts (makes sure)
D
ata
S
et
R
eady (DSR) on J1 pin 6
via resistor R16. Thus, whenever the MFJ- TNC is powered up, it signals to
the terminal connected to J1 that the MFJ-TNC is "on line."
The terminal indicates it is ready to receive data from the MFJ-TNC by
asserting its
D
ata
T
erminal
R
eady (DTR) output, J1 pin 20. The MFJ-TNC
will send data when it has data to send and DTR is asserted. If the terminal is
not ready to receive data, it should negate (make false) DTR to the MFJ- TNC .
Thus, data flow from the MFJ-TNC to the terminal is controlled by the use of
the DTR line. The state of the DTR line is ignored by the software if "software
flow control" is enabled in this direction.
The MFJ-TNC asserts its
C
lear
T
o
S
end (CTS) output, J1 pin 5, whenever it is
ready to receive data from the terminal. If the MFJ-TNC 's buffers fill, it will
negate CTS, signaling the terminal to stop sending data. The MFJ-TNC will
assert CTS when it is again ready to receive data from the terminal. Thus, data
flow from the terminal to the MFJ- TNC is controlled by the use of the CTS
line. The CTS line is always asserted if "software flow control" is enabled in
this direction.
Some serial I/O ports do not implement CTS, DTR and DSR handshaking. If
these pins are not connected at the terminal end, they will be pulled up (and thus
asserted) by resistors at the MFJ-TNC end. However, a non-standard serial
connector may use some pins for other purposes, such as supplying power to a
peripheral device, so be sure that your system either implements the CTS, DTR
and DSR handshake or has no connections to these pins of J1 whatsoever. Note
that reference to RS-232C "compatibility" or the presence of a DB-25 type
connector does not guarantee that you have a full RS-232C serial port!
