This appendix describes the BACnet protocol implementation of the HMD65 relative humidity and temperature transmitter.
- MS/TP Master
Master mode operation means that the HMD65 transmitter is part of a token passing loop. Master can send messages to the MS/TP bus when it has received a token. According to BACnet standard, the master address must be in the range 0 … 127. For correct operation, Master mode also requires that the parameter MAX_MASTER is set. It must be same in every master node connected to the same bus segment. MAX_MASTER defines highest MAC address used by master nodes. For instructions on configuring the MAX_MASTER parameter with Vaisala Insight PC software, see Configuring BACnet with Insight.
Master functionality enables automatic network setup. The client can send inquiries such as “who has property XYZ” or “who is device number 12345”. The device replies to inquiries when it is available to talk. When the master mode device has no token it behaves as a slave, that is, it replies to requests.
The MS/TP address for MS/TP master is 0 … 127. The address is selected by DIP switches: see Modbus and BACnet Overview.
The fixed serial line parameters are: No parity, 8 Databits and 1 Stop bit.
The communication speed is selected by DIP switches. The supported communication speeds are 9600, 19200, 38400, 57600, 76800 and 115200.
If DIP switches are set to select 4800 bps, 38400 is used instead. 4800 bps is not a supported BACnet baud rate.
The physical interface for using HMD65 through BACnet is galvanic isolated RS-485. There are weak pull-up and pull-down resistors (100k) from data lines to +3.3VDC and GND. A termination resistor (120 ohm) can be connected between data lines with a DIP switch on the HMD65 component board. Three screw terminals are needed: D+ (485+), D- (485-) and GND (RS485_GND). For DIP switch and screw terminal locations, see Figure 1.
There are several settable parameters for BACnet communication. Some parameters must be set correct before device is connected to the BACnet network. The initial BACnet configuration of the device is selected when ordering the device, and can be configured using Vaisala Insight PC software, DIP switches, or BACnet protocol. Table 1 lists the configuration options.
|Parameter||DIP switches||Vaisala Insight PC Software||BACnet|
|Device object: Object Identifier|
|Device object: Object Name|
|Device object: Location|
|Device object: Description|
Services are implemented according to BACnet standard. When there are options or the standard is open for interpretations, HMD65 implementation is explained here.
|Protocol Service||Implementation Note|
|Write Property||Reply delay is approximately 225 ms and writing to non-volatile memory may take time close to that.|
|Device Communication Control||Limited functionality. Due to inaccurate internal clock, does not support Device communication control with finite duration.|
|Unconfirmed COV Notification||Latest_sent_value is initialized on startup. If difference between the sampled Present_Value and latest_sent_value is larger than COV_INCREMENT, then COVU is sent and latest_sent_value set to the sampled value.|
Persistence of writable parameter values in BACnet devices is largely a local matter. Nonvolatile parameters are saved in device Flash memory. Volatile parameters are always initialized to some default value at startup. This is defined in each property definition in this appendix. Saving takes effect during the Write Property function.
|Nonvolatile||Value stored in Flash data area|
|Volatile||Value stored in RAM, initial value in Flash|
|Fixed||Value stored in Flash code area|
Additions to BACnet Standard
The BACnet standard allows vendor-specific additions. The following additions are specified for HMD65.