the transmit interrupt is called after the last bit of the character is pumped out.
call txmit interrupt once u have made your sequence to be xmitted in the buffer. maintain the size of buffer in a memory variable. after the first byte is sent thru your prepare transmit buffer is prepared.

start tranmit interrupt thru following:

xmit_size=0;
sbuf = xmit_buff[xmit_size];
xmit_size++;
do_xmit= True ;

serial interrupt proc:

if ( ti & do_xmit)
{
sbuf=xmit_buff[xmit_size];
xmit_size++;

if ( xmit_size >= total_buff_length)
do_xmit = FALSE;
}