Serial Communications; Serial Communication Description; Rs-232 Description; Rs-422/Rs-485 Description - Johnson Controls FRICK QUANTUM LX AcuAir Communications Manual

SERIAL COMMUNICATIONS DESCRIPTION

RS-232 DESCRIPTION

(NOTE: The Q5 board does not utilize RS-232.)
Almost all laptop and desktop computers have
at least one RS-232 serial communications
port available. It was initially developed for the
emerging computer industry in the 1960's. Origi-
nally, it was a method of sending data from a
mini or main frame computer, to devices such as
printers, punch card readers, teletypes, magnetic
tape units and modems. In those early days, the
maximum speed at which RS-232 was capable
of transmitting (about 9600 bits per second),
was quite satisfactory, as most of the receiving
devices were mechanical in nature (except for
modems), and barely able to keep up with these
speeds.
RS-232 uses single ended TX (transmit data) and
RX (receive data). This means a common ground
wire is shared between TX and RX, so only 3
wires are needed for a data only serial channel:
TX, RX, and GND.
Disadvantages of single ended signaling is that
it is more susceptible to noise than differential
signaling (RS-422/485), effective cable distances
are shorter (typically about 50 Ft. total, due to
low noise immunity) and data rates are slower.
Additionally, there is the limitation that only two
devices can communicate together (master and
slave).

RS-422/RS-485 DESCRIPTION

When serial communications started moving
into the industrial environment, it was quickly
QUANTUM™ LX AcuAir™ CONTROL PANEL
COMMUNICATIONS SETUP
SECTION 2

SERIAL COMMUNICATIONS

noted that because of the high electrical noise
potential from electric motors, valves, solenoids,
fluorescent lighting, etc., that the noise immunity
characteristics of RS-232 protocol was grossly
lacking. Additionally, the distances between the
communicating equipment on the factory floor
was much greater than that within the typical of-
fice environment. For these reasons, RS-422 and
RS-485 was developed.
• RS-422 is a full duplex communications
hardware protocol. This means that it data
can be sent and received simultaneously.
Frick® Controls uses a 4-wire system for
RS-422 (two transmit wires and two receive
wires). Advantages of RS-422 over RS-232
is that up to 30 Quantum™ controllers may
be simultaneously connected using a daisy-
chain wiring scheme (to be explained later),
and that the distances involved can be much
greater (typically up to 2000 ft. for the to-
tal cable run), much greater noise immunity
than RS-232.
• RS-485 is a half duplex bus. This means that
it can only send data, or receive data at any
given time. It cannot do both at the same
time. Frick® Controls uses a 2-wire system
for RS-485 one positive transmit/receive
wire and one negative transmit/receive
wire). Up to 30 Quantum™ controllers may
be simultaneously connected up to a total
distance of 2000 ft. using a daisy-chain wir-
ing scheme (to be explained later). One ad-
vantage to using RS-485 as opposed to RS-
422 is that only a single twisted pair cable
need to be run to all devices (while RS-422
requires a double twisted pair cable), much
greater noise immunity than RS-232.
090.510-CS (DEC 2012)
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