5902 Bell 202 Modem Hardware Manual
October 19, 2007
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5.8
M Option
The M option provides the model 5902 with special carrier detect control to improve
communication when using the Modbus RTU protocol. It is recommended for use with personal
computer software packages which experience Modbus RTU protocol errors when a noise burst
occurs at the end of data reception from an RTU.
The M option is recommended when using the 5902 modem by a personal computer to communicate
with TeleSAFE VS/3 RTUs and TeleSAFE 6000/6000-16EX RTUs that use the model 6902
modem. Neither the 6902 modem, nor the TeleSAFE VS/3 modem provide soft carrier turn-off.
Consequently, a noise burst will often occur at the conclusion of each transmission from them.
The M option is not required when all the modems in the communication system implement soft
carrier turn off tones. The soft carrier turn off tone prevents carrier turn off noise.
5.8.1
Carrier Turn Off and the Modbus RTU Protocol
The Modbus RTU protocol determines when a message has been received by monitoring the time
between the receipt of characters in the message. If 3.5 character times elapse without a new
character a message is assumed to be complete. At 1200 baud messages are assumed to be complete
29.2 milliseconds after the last character is received.
Carrier turn off without soft carrier turn off often results in the receipt of a noise character. A noise
character occurring within the framing interval will corrupt the preceding message since the
character is assumed to be part of the message.
If a noise character is received after the 29.2 millisecond framing interval, the preceding message
will not be affected, but the character itself may be interpreted as a new Modbus message. This extra
message may cause problems with some personal computer software. TeleSAFE and SmartWIRE
RTUs handle this additional message without trouble.
The M option provides additional carrier detection control to minimize noise characters and ensure
that Modbus RTU framing is respected.
5.8.2
M Option Operation
The M option affects operation of the model 5902 modem by modifying the clear-to-send hold time
and the carrier loss delay.
5.8.2.1
Clear-to-Send Hold Time
The clear-to-send hold time is always 50 milliseconds. This prevents transmitted carrier from
turning off until 50 milliseconds after the last data character is transmitted. If a noise character
results, it will occur sufficiently past the end of the Modbus RTU message that the message will not
be corrupted.
5.8.2.2
Carrier Loss Time
The modem receiver is disabled 33 milliseconds after the last data bit received by the modem.
The modem receiver remains disabled until the received carrier disappears for a time period equal to
the CL delay, and then reappears for a time period equal to the CD delay. Therefore, any noise
character occurring after 33 milliseconds will not be received.
To ensure a carrier turn-off noise character is rejected, the transmitting modem should turn off its
carrier no earlier than 35 milliseconds after the last character has been transmitted. The default value
