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5.5.3
Modem Receives Data but Never Transmits
The modem receives data without errors but never transmits a reply.
If the transmitting modem is a constant carrier, full duplex master, the receiving modem must be
configured for constant carrier operation. Check the carrier loss switch settings on the receiving
modem. On a 5902 modem the CS0 and CS1 switches must both be closed.
If the transmitting modem is not a constant carrier master, it must be configured for half duplex
operation. Open the FDX switch on the transmitting modem. Check that the transmitting DTE
properly implements the RTS/CTS handshake. See the problem description for RTS Always On in
Half Duplex for more information.
5.5.4
Other Modems Do Not Receive Data
The modem transmits data, but no data are received at the other end.
This problem can occur if a four wire system is not wired correctly. Check that the transmit signal
output is connected to the receive input on the other end. Check that all modems have the 4 WIRE
switch closed and the 2 WIRE switch open, to select four wire mode.
This problem can occur with very short messages, if the transmitter's RTS/CTS delay is too short. It
is possible that the entire message might be transmitted in less than the receiver's carrier detect
delay time. Increase the RTS/CTS delay on the transmitter. If this does not work, decrease the
carrier detect delay on the receiver.
This problem can occur is the signal level is too low. Check the level by putting the modem into the
tone transmission test mode (see section 6.2-Modem Test Modes). The receiving modem's CD LED
should be on, and its RX LED should blink as data is received. If necessary, adjust the modem
transmit level (TX GAIN) to get a steady CD at the receiving modems. On rare occasions, it may be
necessary to adjust the receiver sensitivity (RX GAIN) of the receiving modems.
5.5.5
Bad Data at Start of Message
There are missing or corrupted characters at the beginning of a message. The rest of the
message is good.
If the first part of the received data is missing or garbled, increase the transmitter's RTS/CTS delay
by closing CS0 and/or CS1. You can also reduce the carrier detect delay by opening CL0 and/or
CL1 in the receiving modem.
5.5.6
Bad Data after End of Message
A garbage character appears after the end of a message. All data before the character is good.
If a garbage character appears after the end of a message, use soft carrier turn-off on the transmitter.
On a 5902 modem closing switches SC0 and/or SC1.
If the receiver doesn't recognize soft carrier tones (e.g. a model 6902 TeleSAFE modem or
TeleSAFE VS/3), close CSH and/or SC0 and/or SC1 to transmit a quiet period after each
transmission. The noise burst will still occur, but it will occur later in time, after the end of the
message.
A noise burst at the end of transmission is common and can be difficult or impossible to get rid of
without the soft carrier turn-off tone.
5902 Bell 202 Modem Hardware Manual
October 19, 2007
34
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