robustness improvements in IEC support

c64/kernal/second.s

@@ -1,6 +1,8 @@
 ; Function defined on pp272-273 of C64 Programmers Reference Guide
 second:
 
-	;; This trivial routine is documented in 'C64 Programmer's Reference Guide', page 296
+	;; This trivial routine is documented in 'C64 Programmer's Reference Guide', page 296 and
+	;; https://www.pagetable.com/?p=1031, , https://github.com/mist64/cbmbus_doc
+	and #$1F ; make sure bits encoding the command are cleared out
 	ora #$60
 	jmp iec_tx_command