CTCSS stands for continuous tone coded squelch system, often referred to as "PL" (Motorola's trade name). Many repeaters require the use of a PL tone to access the repeater.
This chart shows each PL tone's two-character alphanumeric designator and corresponding tone frequency in Hertz.
XZ 67.0 1B 107.2 6A 173.8 WZ 69.3 2Z 110.9 6B 179.9 XA 71.9 2A 114.8 7Z 186.2 WA 74.4 2B 118.8 7A 192.8 XB 77.0 3Z 123.0 M1 203.5 WB 79.7 3A 127.3 8Z 206.5 YZ 82.5 3B 131.8 M2 210.7 YA 85.4 4Z 136.5 M3 218.1 YB 88.5 4A 141.3 M4 225.7 ZZ 91.5 4B 146.2 9Z 229.1 ZA 94.8 5Z 151.4 M5 233.6 ZB 97.4 5A 156.7 M6 241.8 1Z 100.0 5B 162.2 M7 250.3 1A 103.5 6Z 167.9 0Z 254.1
Contrary to popular belief, many repeaters that require the use of a specific PL tone to access the repeater are NOT closed repeaters. PL is often used as a means of solving an interference problem, or preventing one in the first place. Some repeaters may also generate a PL tone on the repeater output so that repeater users who are equipped with a radio capable of decoding PL will not hear other interference sources on the channel that would otherwise open the user's radio's squelch.
It is up to the owner / trustee of the repeater to decide whether or not to make public the PL tone for a particular repeater. ARCC follows the wishes of the owner / trustee in publishing the PL code only when so requested.
ARCC strongly recommends the use of PL on repeaters' receivers. PL is a minor inconvenience when you consider how many potential problems it can eliminate. The use of PL may be required for a coordination to be granted if conditions so warrant, such as proximity to a co-channel repeater, or in an area where band openings frequently aggravate co-channel interference problems.
ARCC will soon be publishing a regional PL tone plan in the hopes that repeater owners / trustees in a given area will standardize on a particular PL tone for that region. The reason for this is to make it easier for users to operate the local repeaters as some older radios are only capable of a single PL tone as compared to modern radios which can have PL tones selected on a per-channel basis.