Remote Processing RPC-30 - Manual (Page 15)

CHAPTER 4 SERIAL PORTS
RPC-30 Page 13
Figure 4-2 Network diagram
Only one device on a RS-485 network should have a
terminator installed. The host transmitter shou ld also
have a 100 ohm resistor in series with a 0.1 m fd
capacitor . T he term inator on the RPC -30 includes pu ll
up and pull down resistors to prevent lines from floating
and generating er roneous char acters.
MULTIDROP NETWORK
You can use the RPC-30 in a m ultidrop network by
using CO M2' s RS-422/ 485 port. You can c onnect up to
32 units (including other RPC -30' s) over a 4,000 foot
range.
Figure 4-2 shows an exam ple of a multidrop network.
This networ k includes a host and one or m ore devices.
The host transmits data packets to all of the devices, or
nodes, in the network. A data packet includes an
address, com mand, data, and a checksum. See figure 4-
3. The packe t is received by all devices, and ignored by
all except the one addressed.
The relationship described below between nodes and the
host is a maste r-slave. The host dir ects all
c o mm u ni ca ti on . N o de s " d o n o t s pe a k u n le ss sp o ke n to " .
Peer to peer com munication, while possible with the
RPC-30, is not discussed here. The problem is defining
a bus arbitration scheme, of w hich there are many.
Ther e are m any com municatio n protoco ls. F or this
example, a protocol might look som ething like this:
> 22M B1
The pr otocol starts w ith the < cr> charac ter. This
character synchronizes all units and alerts them that the
next few characters coming down are address and data.
In this case, "> 22" is the units address. "M " is the
comm and and " B1" is the ch ecksum . T he comm and is
terminated with a < cr> character.