Babel Buster 485XL Overview

The Babel Buster 485XL BACnet Gateway supports connecting a LonWorks TP/FT-10 device to a BACnet MS/TP network. It does not support connecting a BACnet device to a LonWorks network. The 485XL gateway expects to be the LonWorks network manager, and should not be used on a managed LonWorks network. Up to 20 LonWorks devices may be connected to a single gateway. The 485XL gateway is a BACnet slave (server) only, and does not include a BACnet client.

Use an Analog Input object to read a LonWorks network variable defined as an output or NVO (Network Variable Output) in the remote device. Use an Analog Input object set to "Out of Service" to write a LonWorks network variable defined as an input or NVI (Network Variable Input) on the remote device.

Use an Analog Value object to provide the most effective write access to a LonWorks NVI. A write to Present Value requires a priority level, and this value is placed into the object’s priority array. The result of processing the priority array according to BACnet standard will be written to the LonWorks NVI in the remote device any time the Present Value is written. To reiterate, the highest priority value is written to the NVI, and the contents actually written are returned as the Present Value read from the BACnet object.

Use a Binary Input to read a LonWorks NVO bit field. If the Binary Input object is placed out of service, it can be used to write to a LonWorks NVI bit field.

Use a Binary Value object to provide the most effective write access to a LonWorks NVI bit field. The Binary Value behaves the same as Analog Value, except that the Binary Value applies to single bit objects. The Binary Value’s use of the priority array is identical to use in the Analog Value.

Communication problems between the gateway and LonWorks device will be logged and reported via the Reliability property of the affected object. Vendor specific Reliability codes indicate “no response”, etc. If the object is in fault as a result of communication problems, the Status Flags will indicate Fault, and the Event State will also indicate Fault.

Babel Buster 485XL Capacities:

Initial Setup or Setup Recovery

The gateway’s station ID, device ID, and baud rate for the BACnet MS/TP port are set by writing properties to the device object. This of cource presumes that you know what they are initially. By holding the Service button down for about 30 seconds to cause an INIT mode reset, you force the station ID to one (1), device ID to one (1), and baud rate to 9600. It is assumed that you will make these one-time device settings while the gateway is not connected to the rest of your BACnet network, and is instead connected to a PC running BACbeat or Control Solutions proprietary Babel Buster XLConfig tool.

General Discussion of Object Behavior

Writeable Input Objects

Input objects are normally read-only. They become write-only objects if placed out of service. Normal operation performs periodic LonWorks polls to read the configured NVO, and place the contents in the Present Value buffer of the BACnet object.

Writes to an Input object will normally not be accepted. However, if the object is placed out of service, you may now write the Present Value. The value written will be written to the configured LonWorks NVI. While out of service, reading the BACnet Present Value will simply echo what was written. To determine if the contents were successfully written to the LonWorks device, you need to check the Reliability property.

Value Objects

Value objects are read-write objects, however, LonWorks network variables are not bidirectional. Objects are assigned to either an NVI or NVO. Value objects provide full priority array functionality in controlling the contents of a LonWorks NVI (input variable at the remote device usually corresponding to a hardware output). A priority level must be included when writing the Present Value of a Value object. The highest priority level not yet relinquished will be written to the NVI each time the Present Value is updated. If all priorities are relinquished, the relinquish default value is written to the NVI when the last level is relinquished. The NVI contents actually written will be placed in the value object’s Present Value buffer for subsequent BACnet reading.

If the object is placed out of service, the Present Value may be written directly, and the result is a direct write to the NVI. LonWorks is treated as write-only by the AV object whether in or out of service.

Scalar Network Variables versus Structures

Most network variables are a single data element such as an integer or floating point value. These single data elements are also known as "scalar" variables types. Some network variables actually consist of multiple data elements organized as a structure usually documented in the syntax of the C programming language. Little harm is done in reading only parts of a structure; however, attempting to write only part of a structure is likely to result in at least some erroneous data being transmitted to the LonWorks device. This problem is addressed in the Babel Buster 485XL by allowing groups of objects to be defined as members of a "group" which make up a structured network variable.

When the gateway sees a BACnet write to a member of a group, all members of the group are gathered up and a complete structure is assembled from all of the various objects in the group. This structure is then written out to the LonWorks device as a single multi-byte structured variable. One of the object properties is a set of flags, and one of these flags is the Group flag. This flag must be set when the object is a member of a group.

There may be some concern about updating all fields of a structured variable before it is actually sent out to the LonWorks device. This is accomplished by setting the Lock flag for one of the members of the group. A BACnet write to non-lock members of the group will simply store the data for subsequent use. A BACnet write to the lock member of the group will result in gathering up all of the stored data for that group and sending it at that time.