Data Sheet

AC4790
Datasheet
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bytes that must be in the buffer for CTS to be disabled (logic High). Even while CTS is disabled, the OEM host can send data
to the transceiver, but it should do so carefully.
Note: The CTS On/Off bytes of the EEPROM can be set to 1, in which case CTS will go High as data is sent in and Low
when buffer is empty.
4.2.4 RTS
With RTS disabled, the transceiver will send any received data to the OEM host as soon as it is received. However, some
OEM hosts are not able to accept data from the transceiver all of the time. With RTS enabled, the OEM host can prevent the
transceiver from sending it data by disabling RTS (logic High). Once RTS is enabled (logic Low), the transceiver can send
packets to the OEM host as they are received.
Note: Leaving RTS disabled for too long can cause data loss once the transceiver’s 256 byte receive buffer fills up.
4.2.5 Test / 9600 Baud
When pulled logic Low before applying power or resetting, the transceiver’s serial interface is forced to a 9600, 8-N-1 (8 data
bits, No parity, 1 stop bit). To exit, the transceiver must be reset or power-cycled with Test pin logic High. This pin is used to
recover transceivers from unknown baud rates only. It should not be used in normal operation. Instead the transceiver
Interface Baud Rate should be programmed to 9600 baud if that rate is desired for normal operation. The Test/9600 pin should
be used for recovery purposes only as some functionality is disabled in this mode.
4.2.6 RSSI
Instantaneous
RSSI
Received Signal Strength Indicator is used by the OEM host as an indication of instantaneous
signal strength at the receiver. The OEM host must calibrate RSSI without an RF signal being
presented to the receiver. Calibration is accomplished by following these steps:
1. Power up only one transceiver in the coverage area.
2. Measure the RSSI signal to obtain the minimum value with no other signal present.
3. Power up another transceiver and begin sending data from that transceiver to the
transceiver being measured.
4. Separate the transceivers by approximately ten feet.
5. Measure the peak RSSI, while the transceiver is in session, to obtain a maximum value
at full signal strength.
Validated
RSSI
As RSSI is only valid when the local transceiver is receiving an RF packet from a remote
transceiver, instantaneous RSSI can be very tricky to use. Therefore, the transceiver stores the
most recent valid RSSI value. The OEM host issues the Report Last Good RSSI command to
request that value. Additionally, validated RSSI can be obtained from Receive Packet and Send
Data Complete API commands and from the Probe command. Validated RSSI is not available at
the RSSI pin. The following equation approximates the RSSI curve, which is illustrated in Figure 1.
Signal Strength (dBm) = (-46.9 × VRSSI ) – 53.9
Figure 1: RSSI Voltage vs. Received Signal Strength