Figure 4.21 Rs-485 Ppt Multidrop Network - Honeywell PPT User Manual

Each PPT on a communication ring must have a unique address in order to individually receive
commands from the host processor. For example, if more than one PPT on a ring has a null address,
and a 00 address command is transmitted by the host, only the first null address PPT will receive the
command and the command is not passed on. This is the same for any address on an RS-232 ring
network.
Another advantage of the RS-232 ring network is the ability to automatically assign device IDs to
every PPT on the network. Since commands flow through every PPT on the ring, a single *99ID=01
command will assign ID=01 to the first unit, and each in the ring will assign itself the next number. As
the command is passed along, each unit adds one to the ID command. For the example shown in
Figure 4.20, the command will read *99ID=07 when it returns to the host processor. The form of self-
addressing will indicate the PPT position in the communication ring relative to the host processor.
Every unit in the RS-232 ring network has an assigned group address. When the host processor
sends a group addressed (*90 to *98) command, that command cycles through every PPT and is
passed to the next one in the loop. Those units having that group address read the command, then
pass the command and reply to the next unit in the ring. This is called a sequential response. Some
sequential responses pass the string of PPT replies before the group command while others pass the
string of PPT replies after the group command. See Table 5.3 for specific sequential command types.
Every unit in the RS-232 ring network receives a global addressed (*99) command. All units read the
command and pass the command and their response to the next unit in the ring. The sequential
response is the same as for the group command described above. See Table 5.3 for sequential
command types.
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 ³120 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 PPTs on a network is 89.
A PPT multidrop network connection of six units is shown in Figure 4.21. 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.
Host Processor
GD - Signal Ground
TD - RS-232 Transmit
RD - RS-232 Receive
A
GD
TD 120
RD
B
120
A - RS-485 (-)
B - RS-485 (+)
Figure 4.21—RS-485 PPT Multidrop Network
2 6
ID=03 ID=05
Group=9303 Group=9102
ID=02
ID=04
Group=9101
Group=9302
ID=01
Group=9103
ID=06
Group=9301