| 摘要 |
A new key telephone system including a new key telephone and new telephone interface which use a new data transmission scheme has been developed, unlike an old key telephone system including an old telephone and old telephone interface unit which use an old data transmission scheme. The new telephone interface unit and new key telephone are compatible with both the new and old data transmission schemes. Even if, therefore, the new telephone interface unit is connected to the old key telephone, or the old telephone interface unit is connected to the new key telephone, data transmission can be performed by using the old data transmission scheme, thus maintaining compatibility.
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