Common Problems And Solutions; Rts Always On In Half Duplex; High Error Rate - Bell 5902 Hardware Manual

5.5

Common Problems and Solutions

Communication using the Model 5902 Bell 202 modem is usually straightforward. The following
list shows problems that can be encountered with possible solutions. If you experience a problem,
which is not described below, please contact Control Microsystems for technical support.
5.5.1

RTS Always On in Half Duplex

The RTS input to the modem is always on, even though the DTE device is configured for half
duplex operation. No data is ever received by the modem.
This problem occurs when the RS-232 device does not implement the proper RTS/CTS hardware
handshake. The RTS signal is always on. Therefore the modem always transmits and cannot receive
data. Other modems can never transmit because carrier is always present.
The RTS/CTS handshake is mandatory, when using the modem with two wire networks or radios.
The DTE device configuration must be modified to correct this problem. If the DTE device does
provide RTS/CTS control, you can purchase the F Option for the model 5902 modem. The F Option
provides RTS/CTS control if the DTE is not capable of providing it.
5.5.2

High Error Rate

Data received by all modems in a system are missing or corrupted at any point in a message.
Entire messages are sometimes lost. There is no pattern in the location of the bad data.
A high error rate usually indicates a poor signal-to-noise ratio; a weak signal that is too close to the
carrier detect threshold; or a signal which is far too strong. A high error rate can also indicate
interference, especially with radio systems. Telephone systems sometimes are noisier when the
weather is poor.
Try to increase the signal level if it is low. If multiple modems are connected to the same wiring, it
may be necessary to reduce the number of modems, or to use conditioned lines. If a radio system is
used, it may be necessary to increase the transmit power, or change the modulation or antenna
characteristics.
A very noisy communication system can cause false carrier detection which produces spurious
garbage character reception. Increase the carrier detect delay. Decreasing the receive sensitivity by
adjusting RX GAIN may also help if the signal-to-noise ratio is sufficient.
A high error rate with radio systems may indicate the audio output level is too high, causing too
strong a received signal. The receive signal should be no higher than 10 dBm (2.45 V RMS) if the
receive gain has not been adjusted from the factory setting. A receive signal of 0 dBm (775 mV
RMS) to –10 dBm (245 mV RMS) is ideal.
For radio systems, confirm that the output signal level matches the radio transmitter's requirements.
Some transmitters may require a very small signal. Adjust the TX GAIN potentiometer, and/or use
external resistors to attenuate the signal if necessary.
5902 Bell 202 Modem Hardware Manual
October 19, 2007
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