User's Manual

FreeWave Technologies Spread Spectrum Transceiver User Manual
900 MHz and 2.4 GHz
V5.0S
56
RS422 & RS-485 Operation
For both RS-422 and RS-485 the Freewave modem can drive 32 standard unit
loads and loads the buss with only 1/8 unit load. This means the user can tie up
to 256 devices on the buss if all of the line receivers have 1/8 unit load (different
devices have different loads).
RS-422 is used for full duplex (4 wire) communication with one master and many
slaves. The Freewave modem is the master unit and keeps the line driver
asserted (in the on state) at all times. The maximum line length is 4,000 feet
using a pair of 120 ohm twisted cables with a 5th wire for data common (neutral).
RS-485 full duplex (4 wire plus common) is the same as RS-422 except the
system can have multiple masters on the buss.
The most common operation of RS-485 is a two-wire (single twisted pair, 120
ohm impedance) system. In this system the loading of the Freewave modem is
as described above which allows up to 256 1/8 unit load units on the buss.
Maximum line length is also 4,000 feet with a third wire required for data
common. Before sending data the Freewave modem will check the line to be
certain no other device is transmitting before it enables its line driver for data
transmission.
When setting the modem to RS-485 for point-to-point mode you must turn
Modbuss on and set master packet repeat to 3 in both the master and slave
radios. In point-to-multipoint you must turn Modbuss on and set master packet
repeat to 1 or more on all radios.
The “Turn off delay” time in the menu is used to control the length of time the
transmitter driver stays asserted (on) after data transmission has finished. This is
needed to allow the last transmitted character to reach the end of a long line
(4,000 feet) and is normally set to one character length of time (four one quarter
character times in the menu) at the selected baud rate. This time also allows 3
complete reflections to the end of the line to ensure the ringing on the line has
fully dampened out before turning the buss over to another device. Shorter line
lengths may use smaller delays if desired but four one-quarter-character delay
times (one complete character) are recommended. A setting of zero delay time is
not allowed as it causes internal timing errors.
There is no provision for hand shaking in any of the above modes of operation so
fast data rates (57.6 K baud and above) are not recommended without a protocol
that can handle error detection properly.