Rs485 Ports - Motorola V186 - Cell Phone - GSM System Planner Manual

IP Ports). The RS232 ports may operate at data speeds up to 115.2 kbps (depending on
the total wire length).

RS485 Ports

On ACE CPU modules, Serial Port 1 (SI1) can be configured as RS485 port. Additionally
up to two RS485 Plug-in ports can be installed on the CPU module (on PI1 and PI2 plug-
in ports).
The RS485 ports permits up to 32 2-wire RS485 devices to be parallel-connected (multi-
drop) onto one pair of wires for the exchange of data. A typical ACE3600 use for RS485
is the interconnection among multiple RTUs in the same site. RS485 is also used to
connect various devices in the site to the RTU using the ModBus protocol or a user
defined protocol. The RS485 Connection Box is available to make this interconnection;
or the installer may make the cables by using the small handset-size connectors
commonly found on modular telephones. The RS485 port may operate at data speeds up
to 460 kbps (depending on the total wire length).
The RS485 specification calls for the circuitry to be capable of communicating at 10
Mbit/s for 40 feet (12 meters). At 4000 feet (1200 meters), maximum cable length, the
data rate is reduced to 100 Kbit/s. There are other factors involved including the network
configuration; wire characteristics, the device used, biasing resistors and termination
resistors (see later) that can influence the data rate. One of the most frequently asked
questions and one of the most difficult to answer is the speed/distance/number of drops
tradeoff.
Different studies in the industry have given some of the following (often conflicting)
results, however the table below provides a conservative estimate based on the
assumption of a daisy chain topology with no stubs.
The following factors affect how far one can reliably transmit at a given data rate:
• Cable length: At a given frequency, the signal is attenuated by the cable as a
function of length.
• Cable construction: Cat 5 24AWG twisted pair is a very common cable type used
for RS485 systems. Adding shielding to the cable enhances noise immunity, and
thereby increases the data rate for a given distance.
Data Rate Distance Distance
(Kbps) (feet)
<100 4000
200 2000
300 1000
400 800
500* 700
99
(meters)
1200
600
300
240
210
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