Little Board™/486e Technical Manual
2–18

Multidrop Network

More than two devices share an RS-485 signal pair , fo r both trans miss ion and recept ion o f dat a.
Only one device is permitted to talk at any one time. As with simple bi-directional communication,
the board’s RS-485 transceiver is placed in receive mode unless it is the one permitted to transmit.
One popular way of managing who is the transmitter is by a “token” passing scheme. Each node is
assigned an ID number. Whoever tran smits also se nds the I D of the n ext n ode allo wed to tran smit.
If a node does not need to transmit, it just immediately sends the “token” to its next node. This
simple scheme is easy to implement and trouble free. Time-outs can be implemented in software to
prevent a lockup should a node fail to pass the token properly.
Serial Console
Ampro’s unique ROM BIOS support for a serial console consisting of a keyboard and display
replaces conventional video controllers, monitors, and keyboards. To use the serial console features,
connect the serial console device(s) to Serial 1 or Serial 2. Use SETUP to configure the
Little Board/486e CPU to use its serial console support feature. The configuration memory stores
serial console configuration parameters.
Caution
Be careful when changing the console configuration using SETUP. If
you specify “None” for console input and output, there is no console
access to the system. (You can recover from this state by removing the
serial console plug from the primary serial port connector and
shorting pins J11-7/8.)
SETUP provides separate configurations for serial console input and outputs (I/Os) so that either
input or output or both input and output are po ssib le fro m an y ser ial p ort an d it’s atta ched serial
device.
To use an ASCII terminal as the console device for your system, set both the I/O parameters to
Serial Port 1 (or 2), and set the serial baud rate, da ta le ngth, and stop bit s to m atch the set ting o f
your terminal. For proper display of SETUP and POST messages from the BIOS, you must use an
IEEE-compatible terminal that implements the standard ASCII cursor commands. The required
commands and their hexadecimal codes are listed in T able 2–14 .
Table 2–14. Required Commands

Hex Command

08 Backspace
0A Line Feed
0B Vertical T ab
0C Non-destructive Space
0D Carriage Return