For the virtual machine language introduced by Infocom, see Z-machine.
Z Code (like Q Code and X Code) is a set of operating signals used in CW, TTY and RTTY radio communication. Actually, there are different sets of Z-codes: one originally developed by Cable & Wireless Ltd. (the Cable & Wireless Service Z-Code), for commercial communications in the early days of wire and radio communications, another one independently developed by NATO forces later for military needs and use. The NATO Z Code is still in use today, and is published in the unclassified document ACP-131. There are other set of codes internally used by Russia's military and other operating agencies. The old C&W Z Codes are not widely used today.
Some Example Z Codes
I am closing down (until...) due to....
Work...(1. Simplex; 2. Duplex; 3. Diplex; 4. Multiplex; 5. Single Sideband; 6. With automatic error correction system; 7. Without automatic error correction system. 8. With time and frequency diversity modem).
Are you receiving my traffic clear?
I am receiving your traffic clear
I am receiving your traffic garbled
Place a competent operator on this circuit
I cannot transmit pictures.
Switch off...(1. IFF; 2. IFF sets for 10 minutes in area denoted except for ships whose call signs follow).
Change to backup frequency no.
Measure of Printability (Where 1=Garbled/unreadable & 5=Perfect)
A lot of the old C&W codes are derived from mnemonics (ZAL = (a)lter wave(l)ength, ZAP = (a)ck (p)lease, ZSF = (s)end (f)aster, etc.