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903-0623-00 Rev. A
Model 903 User's Guide, FMB-X-2.5 Version
5.6.3 RS-485/422/TTL Plug-In (AIB-485)
Card P/N 903-0252-00
The AIB-485 plug-in module, which supports RS-485, RS-422, and TTL, is shown below in Figure 5.6-5.
In addition to the ultra-fast fuses on the AIB-4 motherboard, protection for RS-485/422/TTL inputs and
outputs includes transient voltage suppressors and opto-isolation.
SW2
SW5
SW1
SW3
SW4
Figure 5.6-5: AIB RS-485 Plug-In Module
Each channel has the following possible settings: RS-485 autosense (half duplex), RS-485 unidirectional
transmitter (simplex Tx), RS-485 unidirectional receiver (simplex Rx), RS-422 four-wire connection (full
duplex), or TTL (full duplex).
The equivalent input/output schematic for an RS-422 configuration is shown in Figure 5.6-6, based on
default switch settings. The switches are not shown for clarity. AIB-485 plug-in modules are default
configured for RS-485, in which case the transmit and receive circuits of the RS-422 driver IC are
connected 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 only when it receives 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 act as either a receiver or a transmitter, the data being passed must be sent or
received under timing conditions that allow for collision-free data transmission. (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 generally many
milliseconds. In some cases, though, the autosense timer needs to be adjusted based on the absolute
turnaround time of the external device.
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 5-26
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