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CHAPTER 4 SERIAL PORTS

4-3

Figure 4-2

RS-485 operates in one of two modes: 2 wire and 4

wire. ( An extr a wire is actually neede d as signal

reference. ) There are pro’s and con’s to 2-wire vs 4-

wire systems and they are not discussed here.

Mechanically, to make a 2-wire system, simply connect

T+ to R+ and T- to R- on P3.

When you connect the RPC-2350 to another device, the

T+ ,T-, R+ and R- signal lines may be reversed. That

is, you may ne ed to connect T+ to T- and so on. This

is due to naming convention confusion when RS-485 was

first introduced.

RS-422/485 OPERATING INFORMATION

RS-422/485 Termination network

When the RPC-2350 is the last physical unit on a

network (RS-485), or it is the only unit (RS-422), the

receiver must be terminated to prevent ringing. Insert

jumpers in W5[1-2][3-4] to install the network

terminator. See figure 4-2 below.

Only one slave device on a RS-485 network should have

a terminator installed. The host transmitter should also

have a 100 ohm resistor in series with a 0.1 mfd

capacitor across its T+ and T- terminals. The

terminator on the RPC -2350 includes pull up and pull

down resistors to prevent lines from floating and

generating erroneous char acters.

Two- and four- wire R S-485

The RS-485 port on the RPC-2350 is set up for 2- or 4-

wire mode through jumper W6. This jumper either

always enables receive (W6[2-3]) or enables it only

when the transmitter is off(W6[1-2]).

W6[1-2] Receive off while transmitting (2-wire)

W6[2-3] Receive always on (4-wire mode)

RS-485 Transmitter turn-off

The RS-485 transmitter is automatically turned on and

off by CAMBASIC operating system. You must specify

RS-485 mode in the CONFIG BAU D 2 comm and.

During two wire mode and W6[1-2] is set, the receiver

is turned off when the transm itter is turned on. This

keeps the RPC-2350 from receiving what was just sent

out.

NOTE: Do not operate RS-485 at 150 baud. Automatic

turn off is not adequate. Contact RPC for suggestions.

Figure 4-3 Jumper W 4 & W6 detail

Two-wire RS-485

Mechanically, to make a 2- wire system, simply connect

T+ to R+ and T- to R -. M ake sure CON FIG BAUD is

set up for RS-485 m ode. Se t jumper W 6[1-2]. T his

prevents data transmitted from getting received.

Network response time considerations

When the last character is sent out, an internal timer is

activated. When the timer is done, the RS-485

transmitter is turned off. The timer shuts off the

transmitter about 1/4 to ½ char acter time after the last

character is sent out. The differ ence is due to sto p bit

and parity requirements and to allow for line settling.

Normally, this is not a problem. However, if a very

high speed system is contr olling the netw ork, it is

possible two transmitters can be on simultaneously,

garbling data. Any responding systems on the network

should wait at least ½ character time before sending a

message packet. W hen using other 2350' s on a network,

this will not be a pr oblem at 19200 baud and probably

will not be one at lower r ates.

Multi-drop Network

You can use the RPC-2350 in a multi-drop network by

using CO M2' s RS-422/ 485 port. You can c onnect up to

at least 32 units (including other RPC-2350's) over a

4,000 foot range.

The host transmits data packets to all of the devices, or

nodes, in the network. A data packet includes an

1 2 ... 17 18 19 20 21 22 23 24 25 26 27 ... 82 83

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