Modbus Read, Write and Override Parameters plus Exception Codes
348 UDC3500 Universal Digital Controller Product Manual 3/07
Analog Parameters
• Whenever analog register addresses xx01 through xx7F (those that can be changed
via communications) are changed, a Write cycle occurs after receipt of the message
and the response is returned.
Override Parameters
• Override analog register address xx7D (computer setpoint) is not stored in non-
volatile memory. It can be changed as frequently as desired with no effect on non-
volatile memory retentivity, but the controller must remain in the slave mode.
Digital Parameters
• Whenever digital configuration register addresses xx80 through xxFF are updated via
communications, the non-volatile memory is updated as soon as the message is
received.
Communications Transfer Rates
• The Host Computer must allow a minimum of 20 milliseconds between Read
transactions and a minimum 200 milliseconds between Write transactions.
Supported Function Codes
• IR port 20 and 21
•
RS485 and Ethernet ports 1,2,3,4,6,8,16,17,20,21
Communications Modes of Operation
• When the Shed Timer is enabled and a write or override occurs the controller will
enter Slave Mode. The keypad is locked from the operator. The purpose of this mode
is that if communications is lost and the shed timer times out then the controller will
enter a known state of operation. The configuration of the “Shed Mode and Output”
and Shed Setpoint Recall are used to configure the controller’s shed state. While in
Slave Mode pushing the
Man
Auto
Man
Auto
Man
Auto
key enters Emergency Manual mode. The local
operator then has control of the output. The controller is in Monitor Mode if the Shed
timer is disabled.
EEROM Access
• All setpoints and configuration values are maintained in EEROM (Electrically
Erasable Read Only Memory). To prevent unintended controller operation, the
setpoint and configuration values stored in EEROM may only be altered by one
source at a time, either via the Keyboard or via one of the Communications Ports (IR,
RS-485 or Ethernet). Keyboard alterations take priority over all other
communications methods. Therefore, whenever an operator initiates any change of
value to a setpoint or to any other analog configuration value via the keyboard, the
controller will then respond with a BUSY exception response to any MODBUS
WRITE communications transaction initiated by the Host Computer until the operator
completes accessing the EEROM. This only affects WRITE commands, READ
commands will still be processed normally.
Also, if the operator changes a setpoint value via the keyboard but does not save this
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