zs3: silo overflow ??

[Patel Maqbool 387-1784]

Some time I see following message on my console :  

May 30 12:03:35 cvlsun1 last message repeated 7 times
zs3: silo overflow

What doese it mean ? How do I correct it ?  I know zs0 and zs1 correspond
to the two serial lines (am I correct?).  What is zs3 ?

Thanks in advance.
Patel Maqbool
patel at cvlsun1.ece.drexel.edu