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903-0623-00 Rev. A
Model 903 User's Guide, FMB-X-2.5 Version
5.3.2 Configuration Settings
Channels 1-3 have the following possible settings: RS-485 autosense (half duplex), RS-485 unidirectional
transmitter (simplex Tx), and RS-485 unidirectional receiver (simplex Rx). Channels 4 and 5 additionally
have the option for RS-422 (full duplex, four-wire connection) and Channel 5 has the option for full duplex
TTL. Configuration settings for the switches and jumpers are shown in Table 5.5-3. For DIP switches, a
"1" indicates a switch is "on" or "closed" and a "0" indicates a switch is "off" or "open". For 2-pin jumpers,
a "shunt" indicates the pins are connected with a shorting terminal and "open" indicates the shorting
terminal is removed. For 3-pin jumpers, the table entry indicates which pins are shorted together.
The RS-485 autosense mode uses a timer circuit to automatically switch from transmit to receive mode.
By default, a channel in autosense mode is a receiver waiting for data to come in through the front panel
and switches to a transmitter when it gets data from the backplane. Once the RS-485 channel is in
transmitter mode, it will wait ten bit times (one start bit, eight data bits and one stop bit) from the last
positive data edge before reverting back to its default receiver state.
This half-duplex mode operates in a ping-pong fashion that must be supported by the end equipment.
Although the circuit can sense data at a certain data rate and be either a receiver or a transmitter, the
data being passed must be sent or received under timing conditions that inherently avoid collisions. (If a
data collision does occur, transmission out of the front panel connector will override incoming data.)
Autosense settings only affect half-duplex operation. Default settings for the autosense timer (9600 baud)
are appropriate for most sonars, even when the sonar is operating at higher baud rates, since delays
between sonar send and receive are typically many milliseconds.
A channel configured in simplex Tx or simplex Rx is a two-wire interface that is only designated to
transmit or receive data. Tx is defined as Model 903 transmitting data out the front panel whereas Rx is
defined as the Model 903 channel receiving data from an external device.
Focal Technologies Corp.
Page 5-9
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