Bell 5902 Hardware Manual page 40

for the S4 register (transmit gap time) in the TeleSAFE and TeleSAFE VS/3 is 8 characters (66.6
ms) which is sufficient.
It is essential that the received carrier actually disappear (unless the modem is in half duplex,
constant carrier mode) or the modem receiver will be permanently disabled. For this reason the M
Option is not recommended for both ends of a full duplex link. (Since carrier is never turned off, the
M Option is not needed anyway). The M Option is intended for use in a half duplex switched carrier
or constant carrier system.
5902 Bell 202 Modem Hardware Manual 39
October 19, 2007