Hi Erik,

I can not see any problem or big circumstance to do it with the 8051 build in UART.
The additional interrupts only 6 cycles fast (EX1, ET1).
And the additional code "waste" only 22 bytes.
Following the example routines:


Peter

        dseg
Idle_timer:     ds 1

        bseg
Bus_idle:       dbit 1

        cseg

        org     13h
intex1:
        clr     ET1
        clr     EX1
        reti

        org     1Bh
intt1:
        djnz    Idle_timer, it11
        clr     ET1
        setb    Bus_Idle        ;or AJMP to the handler
it11:   reti

        org     23h
intuart:
        jnb     RI, iu1

        mov     Idle_timer, #4 * 16     ;wait 4 bit times
        setb    ET1
        clr     IE1
        setb    EX1

        clr     RI
        ; do something
        reti

iu1:    ; do something
        reti