Guardian SCC 25, Manual

Page: 63 / 79

Page 63
GUARDIAN SCC-40 Issue (Auto VI) & V3.6t 07/12/00
COMMUNICATIONS
The MASTER unit is always the one associated with the compressors with the least hours
run which is also available for operation in REMOTE suction pressure control mode.
Remote monitoring, modification and control of the compressor system via a second
two-wire serial link (RS485) may be provided using the GUARDIAN Autograph Monitor
Terminal. This IBM compatible PC terminal provides displays of latest values, control and
alarm settings, timers and compressor status together with daily printouts of pressure and
temperature graphs, alarm and trip messages.
Communication facilities are available for interrogation of temperatures, status and
modification / display of setpoints, limits and timeclock settings. All communication is via a
daisy chain RS485 link which connects all GUARDIAN controllers units in series.
Communication commands and replies are checked for parity and block length and
automatically re-transmit if errors are detected.
Each GUARDIAN controller has a unique unit number address UU/u which is used to select
the appropriate unit for interrogation or modification.
UU is system no. 1-80
u is compressor number 1-6.
i.e. compressor 3 system 56 has address 56/3
Some communication commands may use 'wildcard' stub number 99 and 'wildcard' case
number 9 to access all stubs on the highway or all cases in a stub.
GUARDIAN controllers are inactive until they are addressed.
When the organisation of commands on the RS485 highway is under the control of a
Woodley Mk V then GUARDIAN units only accept status requests which transmit case,
discharge and return air temperatures and defrost status.
GUARDIAN Autograph or RM-256 Refrigeration Monitor Communication commands
available are:-
a) Transmit Unit Status which replies with command plus stub status & case
temperature
b) Transmit Values which replies with stub address plus latest signed temperature
values, time, trip states, relay states and internal status
c) Transmit Setpoints which replies with setpoints and limits.
System Sn and unit Addresses Axxx may not be changed via
the RS485 serial link
d) Receive setpoints with new setpoint values
e) Receive Time and Date with new hours and minutes, day, month and year for real time
clock