Advertisement
ADS-14221, PPT2 User Manual
RS-232 Ring Network
The RS-232 network consists of a three-wire bus (TD, RD, and GD) that begins and ends at the host
processor. In general, the RS-232 electrical standard requires that the distance between units not
exceed 60 feet (18 meters). An advantage of the RS-232 network is that it interfaces directly to the
serial COM port of most personal computers (or USB to serial adaptor). The maximum number of
PPT2s with assigned device IDs on a network is 89.
A PPT2 ring network connection of six units is shown below. In this example, the Device IDs
are sequential, starting from 01, from the transmit port of the host processor around the loop. There
are two address groups, 91 and 93; each have 3 PPT2 units assigned.
GD = Signal Ground
TD = RS-232 Transmit
RD = RS-232 Receive
Each PPT2 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 PPT2 on a ring has a null
address, and a 00 address command is transmitted by the host, only the first null address PPT2 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 PPT2 on the network. Since commands flow through every PPT2 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 network
shown above, the command will read *99ID=07 when it returns to the host processor. The form of
self-addressing will indicate the PPT2 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 PPT2 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. Some responses pass the string of PPT2
replies before the group command while others pass the string of PPT2 replies after the group
command. See Table 1.0 for specific command types and the order of repeating the group command.
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.
RD
TD
ID=01
Group=91
GD
TD
RD
TD
ID=06
Group=93
Signal Ground
RD
TD
ID=02
Group=91
Signal Ground
RD
RD
TD
ID=05
Group=91
October, 2016
RD
TD
ID=03
Group=93
RD
TD
ID=04
Group=93
19 of 85
Advertisement
