-9-
STANDARD COMMANDS
Information coming
serial command(s)
Com mand
Response
Notes
Annunciator status
>(01 RD)
<(01 XX)
(XX - Refer to section 6.0)
Read hourmeter
>(01 RH)
<(01 XXXXX)
(XXXX X = Hours)
Read current RPM
>(01 RP)
<(01 XXXX)
( XX X X = R P M )
Reset the annunciator
>(01 RR)
<(01 RR)
Stop the annunciator
>(01 RS)
<(01 RS)
Valid Response - A command/response sequence is not complete until a valid response is
received. When a slave unit receives a valid command, it interprets the command, performs
the desired function and then communicates the response to the master within 20mS. The
master may not initiate a new command until the response from a previous command is
completed.
A valid response can occur in three ways:
1.
A normal response indicated by a "< " header and "( )" beginning and end of text.
2.
An error response indicated by a "§" NAK (not acknowledged) .
3.
A communications time-out error.
An NAK error response will be sent by the annunciator when it has received a command with
an error in the message. All commands must be of the format above. The header,
start-and-end of text characters, a valid node number and spaces must be sent and correct
to receive an NAK; if not, no response will be sent.
8.7 RS-485 COMMUNICATIONS
A.
HALF DUPLEX OPERATION - The RS-485 system employed uses two wires for
communication and cannot send and receive data at the same time over the same two
wires making it a half duplex system. When the master is in the transmit mode, the
slave is in the receive mode and visa-versa.
B.
ELECTRICAL OPERATING RANGE - RS-485 is a communications standard to satisfy
the need for multi-dropped systems that can operate at high speeds over long distances.
RS-485 uses a balanced differential pair of wires switching from 0 to 5 volts to
communicate data. RS-485 drivers can handle common mode voltages from
!
7 to +12
volts without loss of data, making them an excellent choice for industrial environments.
C.
COMMUNICATIONS WIRING - The RS-485 wiring diagram illustrates the wiring
required for multiple slave unit hookup. Note that every slave unit has a direct
connection to the master. This allows any one slave unit to be removed from service
without affecting the operation of the other units. Every unit must be programmed with
a unique address or node number, but the addition of new units or nodes can be in any
order. To minimize unwanted reflections on the transmission line, the bus should be
arranged as a trunk line going from one module to the next. Random structures of the
transmission line should be avoided. Special care must be taken with long busses (500
feet or more) to ensure error free operation. Long busses must be terminated with a 120
ohm resistor between the terminals marked RS-485 "A" and RS-485 "B" at the master
only. The use of twisted pair shielded cable will enhance signal fidelity and is
recommended. To prevent ground loops the shield should be connected to the shield
terminal at the master only.