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HPB\HPA User Manual
RS-485 Multidrop Network
The RS-485 network consists of a two-wire bus (A and B) that begins at the host processor and ends at the far
end from the processor. A star network configuration can also be used where each unit is connected directly to
the A and B terminals at the host processor. An advantage of the RS-485 network is that it can extend up to 4,000
feet (1,220 meters) and units can be added and removed without breaking the network connection. A requirement
of the RS-485 electrical standard is that the bus is terminated at both ends by a 120 ohm resistor. Also, the
standard allows a maximum of 32 units on a single two-wire bus. To accommodate more than this, some of the 32
units can be repeaters. Each repeater can add an additional 32 units. The maximum number of addressable
HPBs on a network is 89.
An HPB multidrop network connection of six units is shown in Figure 4.13. In this network, the device IDs are not
in order from one end of the network to the other. Looking at all the units on the bus, the device IDs are sequential
beginning with ID=01 and without duplication. This is an important setup condition if global commands (*99) are
desired.
This gives the units on a multidrop bus a pre-assigned order to follow when they are responding to a global (*99)
command. Each HPB on the communication bus must have a unique address in order to individually receive
commands from the host processor.
When an RS485 unit has a null address (unassigned), it will only respond to null addressed (*00) commands.
The command response will begin with the ―?00‖ reply header. If two or more null addressed HPBs on a multidrop
network receive a null addressed command, they will all respond together and cause a garbled message.
Although all null addressed units will listen to group or globally addressed commands, and take the appropriate
action, they will not send a response to a group or globally addressed command. It is important to assign each
unit a unique and sequential device ID to begin any useful communication.‖
Assign Device IDs: Start with a list of all the units serial numbers. For example, send the global commands
*99WE and *99S=00001234. This will prepare the HPB with serial #00001234 to assign itself the number from the
next global ID command. Send *99WE, *99ID=02, *02WE, and *02SP=ALL to complete the device ID
assignment. Repeat this for each unit on the RS-485 network. Other methods for device ID assignments are
described in the ID command description.
Assign Group Numbers: Once the device IDs are set the group IDs can be assigned. Each unit in the same
group must have a sequential sub-address - beginning with 01. The sub-address tells each unit what order it
should respond in a group addressed command. To set the group (91) and sub-address (01) for device ID=02,
send the following commands: *02WE, *02ID=9101, *02WE, *02SP=ALL. Now when a command is sent to group
91, device ID=02 will be the first one to reply. In Figure 4.13, there are two address groups, 91 and 93; each have
3 HPB units assigned.
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Figure 4.13—HPB RS-485 Multidrop Network
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