You need a R: handler to access a RS232 port.  The port would have to
be on a 850, PRC, MIO, Black Box, which are much more expensive
than a SIO2PC adapter.  You probably need a disk drive as well as
the 850 R: handler needs to load at boot.  It may be possible to
write the PRC handler in BASIC DATA statements and POKE
it in from a BASIC program, that may work.
So, let's say you have a PRC, can load the R: handler from BASIC.
Then the following program would be OK at 300 baud to operate
your null modem.
******************************
10 CLOSE #2:OPEN #2,4,0,"K:":REM open K: for read
20 CLOSE #3:OPEN #3,13,0,"R:":REM open R: for concurrent read/write
30 CLOSE #4:OPEN #4,8,0,"E:":REM open E: for write to screen
40 XIO 34,#3,192+48+3,0,"R:":REM turn on DTR,RTS,XMT
50 XIO 40,#3,0,0,"R:":REM Open R: for concurrent mode
60 XIO 36,#3,0,0,"R:":REM MUST BE 300, PROG. TOO SLOW FOR 1200 OR
HIGHER
100 STATUS #3,AVAR:REM causes 747 to retrieve from R:, I think, not
sure
110 IF PEEK(747)=0 THEN 200:REM number of chars in input buffer
120 GET #3,CHAR:REM if 747 <> 0 then get the char
122 IF CHAR=0 OR CHAR=10 THEN 200:REM discard ASCII 0 and 10
124 IF CHAR=13 THEN CHAR=155:REM translate ASCII linefeed to ATASCII
130 PUT #4,CHAR:REM send char to E:
200 IF PEEK(764)=255 THEN 100:REM if 255 then no key been pressed
210 GET #2,KEY:REM key pressed, get the char
212 IF KEY=155 THEN KEY=13:REM if linefeed then translate to ASCII
220 PUT #3,KEY:REM send char to Rn:
240 GOTO 100:REM start all over
250 REM R: is for R1:, you could also specify R2:, I think 