8.3 Host/Client (Broadcast) Communications

A simple but very powerful Host Communications Retransmission facility is provided that allows the construction of simple multi-zone temperature control systems. This uses the Modbus broadcast facility to retransmit values to other instruments ('clients') so that, for example, they may be sent a programmed setpoint profile from the 'host' programmer device. It is also possible to use the facility to transmit output power demand to other devices, for example a phase angle thyristor power controller.

 

The broadcast sends 'scaled integer' values, which are integer representations of a floating point number with the decimal places removed. For example, a value of 12.3 would be sent as 123. It is important, therefore, that the receiving and transmitting units are set to use the same decimal resolution. Modbus Function 6 is used for the broadcast, and so the receiving device must be support this function. The Modbus register address that the values are sent to is completely configurable between 1 and 9999.

 

Modbus Broadcasts do not permit feedback from clients, but it is possible to use relays on the client devices to close a contact closure digital input on the host. This can be used with a deviation event alarm and run/hold logic input, for example, to detect when the temperature measurements in the client have not reached the programmed setpoint and to put the program in hold.

 

The retransmitted parameter can be selected from Setpoint, Process Variable, Output Demand or Error. The controller will cease broadcast when it receives a valid request from a Modbus host - this allows iTools to be connected for commissioning purposes.

 

Warning

In common with most instruments in its class, the 800 Range uses a non-volatile memory with a limited number of specified writes. Non-volatile memory is used to hold information that must be retained over a power cycle, and typically, this includes setpoint and status information, including alarm latch status.

 

Please ensure that parameters which do not require updating on a regular basis (for example, setpoints, alarm trip levels, hysteresis, etc) are only written to when a change in the parameter value occurs. Failure to do this could result in permanent damage to the internal EEPROM.

 

When using the 800 Range, use the ‘AltSP’ variable at Modbus address 26 if you need to write to a temperature setpoint. This has no write restrictions and may also have a local trim value applied using the ‘SPTrim’ parameter at Modbus address 27.

 

A further explanation is given in section 8.4 ‘EEPROM Write Cycles’.