Page 141 - Making PIC Microcontroller Instruments and Controllers

132 sERllL coMMuNlcATlOrlS: SOCKETS U9 AllD Ul0

with
(Program
l::Ptdiiallig-ili: Rs232 Communications lo communicale a computed
lcontinued)
RETI'RN qo back Lo calLer
subroutine to send a character
CHAROu!: to usarl Lransnilter
IF PIR1.1T = O THEAI CITAROu! wait for transnit registe! enpty
TXREG = Bl send characLer io transnit

REAURN go back to caLLer
EIID

jfyou
of
On the other hand, need a greatcr understanding what is going on, read on
TheLAB-XIlelsus experiment with twotypes ofserial co munications The board
comes with hardware for the RS232 standard on board and an emply socket that can be
coniigured with the RS485 protocol (a line driver lC musl be ndded). Only one type of
can be actve at any one time, andlhcchip ihatis not being used must
communications
be removed from the board. The RS232 communicalions are routed to the DB 9 female
conneclor on the boa]d,.md there are PC boa]d holes fot s. three'pin connector at Jl0
The IC rcquired by the RS485 is the SN175176A or equiv-
&rRS,l85 communications.
alent line driver.
The two staDdards are simil and, simply stated, using RS485 allows you io go
longer distances and the communicaiion is more noise tolerart. (This is related io the
capacitance of the lines used and slronger drivers being employed. The discussion of
the why and how ofthis is beyond the scope of this book.)
The compiler suppots communications to both standards and the specified compiier
conmrmds should be used whenever possjble. Writing your own sequences lbr control-
even though it might be inslructive
ling serial communications is couDterproductive,
and the hardware you
The compiier uses the same commands to access both standards,
put in place determines how the signaLs are senl o!r! and received. (Take a minute to look
at Figures 8- l and 8-2 to see what the complications ar.J.)
Coimunications arc timed according to the speciiication of the oscillator For the
proper timing to be achieved, the OSC command has to be sct to the actual oscillator
frequency in use. Ifthe ffequercies are not matched, conmunications will be sped up
or slowed down (in spced) based on the extent ofthe mismatch.Ilyou are actudly using
will slot!
a 4 M Hz osci llator and specify OSC 20 in your program, ihe communications
down to onlr fifth thespecified spccd because the system is actuallyrunning at one-fifth
the 20 MHz speed.
In order to experiment with colnmunications, we need to bc able to communicate with
an externaldevice. The easiestdevice use is apersonal computcr set up with a dumb
to
terminal program. The various versions of Microsoft Office Works software all contaln
a tefminal program that you can access and use.
Adumb terminal may be set up by following these instructions:

Co to the St?u]t menu in your PC command lrough, then select