I modified the basic program to wait until ACK goes low, printing a dot every time it checks and the high bit is set. It's interesting that when it's printing that the acks doesn't finish right away.
NEW
1 GOTO 90
2 POKE PO,C : ? C" ";:X = PEEK(PI)
3 IF X < 128 GOTO 5
4 ?".";:X=PEEK(PI):GOTO 3
5 ? : RETURN
90 C=PEEK(49152+256) : REM READ FROM C100 TO ACTIVATE AUTOSTROBE
100 PO = 49152 + 9*16 : PI = PO+4
105 FOR J = 1 TO 24
106 C=64+J : GOSUB 2 : Z$=STR$(J) :GOSUB 3000
110 C = 27:GOSUB 2 : C = ASC("L") : GOSUB 2
111 C = 127:GOSUB 2
112 C = 0 : GOSUB 2
120 FOR I=0 TO 127 : C = I : GOSUB 2 :NEXT
150 C=13:GOSUB 2:C=10:GOSUB 2
160 NEXT
1999 END
3000 FOR Z = 1 TO LEN(Z$) : C=ASC(MID$(Z$,Z,1)): GOSUB 2 : NEXT : RETURN
RUN
It sends the 13 and 10 (CR LF)
then ESC L
then num of chars as two bytes
and you can see the dots get very long there, probably very likely to drop chars there if you're not checking the ACK.