Effective.June.2007
Page 5
RS-485 PONI
66A2070 Rev. B
•
121Ω Termination Dip Switch
.
The 2-ohm termination DIP switch, when switched to
the ON position, places a 2-ohm resistor between the
“A” and “B” terminals. This resistor should be used as an
end-of-line terminator.
•
Address Selector Switches
.
There are two rotary switches that are used to set the
Modbus address on the RS-485 PONI. An address from 0
(zero) to 99 decimal can be selected.
•
Baud Rate Selector Switch
.
The baud rate DIP switch is used to select the RS-485
PONI baud rate. It can be changed at any time and does
not require the cycling of power.
RS-485 Baud Rate Switch
Baud
A0
A
1,200
Open
Open
9,600
Open
Closed
19,200 Closed
Open
Figure 10: RS-485 Baud Rate Switch
Modbus Network Communications
Protocol
Function Codes
The RS-485 PONI responds to a limited number of Modbus
functions codes: 03, 04, 08, and 6 (0
16
) .
Command/Data Pass-Through
The RS-485 PONI can pass host product commands/
data directly through to any one of the attached host
products. Thus, with access to IL 7384 – IMPACC System
Communications, Parts A, B, D, and F, every INCOM product
object and capability is available to the Modbus master.
IL 7384 is available at http://www.eaton.com.
When passing a command or data through to a host
product, the RS-485 PONI acts as a dumb slave. Without
modification, it passes the command or data through to the
product .
In the event the product responds, the RS-485 PONI saves
the response until the Modbus master queries for that
response. The RS-485 PONI makes no modification to or
interpretation of the product response data.
The Modbus master writes the INCOM product command/
data using function code 6 (0
16
) beginning at register
424577.(6000
16
) .
The data format for passing information through the RS-485
PONI to an host product is listed in Figure on Page 6.
The Modbus master reads the INCOM product response to
a pass-through query using either function code 03 or 04
beginning at register 424833 (600
16
) .
The number of points (registers) of the read query is
2*nn, where nn is the number of INCOM messages in the
response .
The format of the data acquired by the RS-485 PONI from
the pass-through INCOM product query’s response is
displayed in Figure 2 on Page 7. Note that each INCOM
response message contains a status byte which indicates its
validity .
Control of Host Product
Since.a.control.error.could.result.in.unwanted.actions.
initiated by an host device, the RS-485 PONI requires a
specific protocol by the Modbus master in order to perform
control-related functions within the host product.
A set of registers is reserved for the control protocol. They
begin at register 425089 (6200
16
) and extend through
425091.(6202
16
) .
These three registers are written with a “slave action
number” and its ’s complement using function code 6
(10
16
). The format of the data is shown in Figure 3 on
Page 8. These three registers exclusively must be written in
one.Modbus.transaction .
If the slave action number and its ’s complement are
valid, the RS-485 PONI issues the slave action control
command onto the host product. If the slave action request
is successfully acknowledged by the product, the RS-485
PONI.returns.a.normal.function.code.16.(10
16
).response.to.
the Modbus master.
The Modbus master may further determine if the product
completed the slave action function successfully by
interrogating the product, for example, by reading its status.
If the host product does not acknowledge the slave action
request, the RS-485 PONI returns an exception code 04.
If the slave action number and its ’s complement are
invalid, the RS-485 PONI responds to the Modbus master
with a data value illegal exception code 03. Refer to the
Exception Codes section on Page 6 for more information.
