Despatch Protocol Plus Modbus Communications - Manual (Page 24)

Chromalox Instruments and Controls
A-51643 Rev. 6 10/06/03 18
4.6 Event Setup Values
For Manual, Timer, and Profile events setup, the corresponding holding register represents a binary
pattern that determines each event output state. Bits 0-3 of the holding register value represent Event
outputs 1-4 respectively. A bit value of 0 sets the event output to be off, while a bit value of 1 sets the
output to be on.
4.7 Keypad Lockout
The front panel keypad can be disabled (locked out) by writing the value 1 to the Keypad Lockout
holding register ($0005). A write of 0 will unlock the keypad. The keypad lockout register is always set
to 0 during a power-up or reset of the controller to ensure that the keypad can never be permanently
locked out.
4.8 Temperature Values
Temperature values read from input and holding registers and written to holding registers are always in F
and assume no decimal place. An exception is the ambient temperature input register ($0090) which
assumes 1 decimal place. A value read or written as 750 would represent 75.0 F.
4.9 Time Values
Time values are read and written in a format that adheres to the to Modbus standard of two bytes per data
item. The high byte is a hex number representing hours or minutes while the low byte is a hex number
representing minutes or seconds. The units of the two bytes (hhmm or mmss) are dependent of the format
of the specific register being accessed. Example: 12h00m $0C00; 36m42s $242A.
4.10 High Limit Setpoint and Band Registers
The Despatch Protocol Plus has several special cases involving the adjustment and display of high limit
parameters. Since only values can be returned via Modbus (no special strings), the read/write of these
parameters has been implemented in the simplest manner that still allows for complete functionality.
All high limit setpoint and band registers read and write directly to the parameter associated with the
register address—regardless of whether that value is active due to the setting of another high limit register
(as in the high limit band holding register $0018). To determine the high limit setpoint currently being
used for control, read the Active High Limit Setpoint register $0002.
4.11 Broadcasting with Guaranteed Delivery
Broadcast messages are processed by all slaves and do not generate a response. In order to verify the
delivery, acceptance, and processing of broadcast messages the following procedure can be used.