I can't aswer the question about how much of the zs0a and zs0b are
generated by the keyboard i.s.o. the serial port cause i communicate with
them mainly through the ethernet with telnet i.s.o a real keyboard&monitor.
And there main bussiness is taking care of routing and ppp so i don't need
to interact much.

But i can give an idee about how many are caused by the ppp use of the
serial port. Hope this gives an insight..

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
kern.osrelease = 1.1
kern.osrevision = 199306
kern.version = NetBSD 1.1 (GENERIC) #47: Wed Nov 22 19:17:19 PST 1995
    root2@wormhole:/disk2/src/sys/arch/mac68k/compile/GENERIC
hw.model = Apple Macintosh IIsi  (68030)
hw.ncpu = 1
hw.physmem = 5046272
hw.usermem = 2605056

gateway from Dec  6 12:00 till Dec  6 23:21

/netbsd: zs0a: ring overrun                           2
/netbsd: zs0a: fifo overrun                        2207

/netbsd: ppp0: bad fcs <xxxx>                      4199
/netbsd: ppp0: bad protocol <xxxx>                    5
/netbsd: ppp0: missing UI (0x3), got 0x80             1

modemport on 38k4 with 19k2 ZyXEL 24hour connection
max. ftp-throughput now is 3.1k/s
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
kern.osrelease = 1.1
kern.osrevision = 199306
kern.version = NetBSD 1.1 (GENERIC) #47: Wed Nov 22 19:17:19 PST 1995
    root2@wormhole:/disk2/src/sys/arch/mac68k/compile/GENERIC
hw.model = Apple Macintosh IIcx  (68030)
hw.ncpu = 1
hw.physmem = 16777216
hw.usermem = 13766656

ppp-server1 from Dec  4 21:00 till Dec  7 00:05

/netbsd: zs0a: ring overrun                           2
/netbsd: zs0a: fifo overrun                        1131

/netbsd: ppp0: bad fcs <xxxx>                      1211
/netbsd: ppp0: bad protocol <xxxx>                   48
/netbsd: ppp0: missing UI (0x3), got 0x<xx>         146
/netbsd: ppp0: VJ uncompress failed on type comp    702
/netbsd: ppp0: garbage received: 0x<xx> (need 0xFF)  15
/netbsd: ppp0: bad char 10000<xx>                    24

modemport on 38k4 with 28k8 Supra dialin connection
max. ftp-throughput now is .9k/s
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
kern.osrelease = 1.1_ALPHA
kern.osrevision = 199306
kern.version = NetBSD 1.1_ALPHA (GENERIC) #0: Tue Nov 21 13:00:36 PST 1995
    root@news:/usr/src/sys/arch/mac68k/compile/GENERIC
hw.model = Apple Macintosh IIx  (68030)
hw.ncpu = 1
hw.physmem = 5242880
hw.usermem = 3088384

ppp-server2 from Dec  4 00:00 till Dec  7 00:09

/netbsd: zs0a: ring overrun                           4
/netbsd: zs0a: fifo overrun                         753

/netbsd: zs0b: ring overrun                           4
/netbsd: zs0b: fifo overrun                         809

last message repeated* <xx> times                   620

and pppd's options /wo debug kdebug 1 ((

2 modemports on 38k4 with 14k4 Supra dialin connections
max. ftp-throughput now is .9k/s
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
cheers,
~n