3 wires acia communication on apple2

["sfbong" <sfbong@tm.net.my>]

 Hi all,		

 I have build a small Single board computer out of 6502A and would like to
 link it to my Apple II+ using  serial interface.  So I build two simple
ACIA 
 boards using 6850, one attached to the SBC and one to my 
 Apple using slot #2.  I only use 3 wires for their communication namelt
TX,
 RX and Ground.

 The speed is 2400 B/S and both sides using Polling method.  My problem was
 the first few bytes are always wrong when I sent a command to hex dump 
 the monitor ROM of the SBC from my Apple.

 I have a frequency counter, and I make sure the Freq at the TXclk and
RXclk of the
6850 was 16 times of 2400Hz and the acia was set up to 1 start, 8Bits,
2Stops 
and divide by 16   i.e.

lda #03, sta aciacr, lda #%00010001, sta aciacr
 
 Was it due to the baud rate was too high?
 Or the 6850 on the SBC must be interrupt driven?
 Or I must add the rest of the RS232 wires cts, rts, dcd ....etc so it
could work.
 
 Any idea on how I can solve my problems?
 
 TIA
 
 Allen Bong