BusWorks
Model 903MB/902MB/901MB Network I/O Module User’s Manual Digital I/O
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Model 903MB-0900 Register Map…continued
Ref
Addr.
Description
Data Type/Format
Holding Registers (4x References, Read/Write)
4
0009
8
(0008)
Port 2
(CH 8-11)
Timeout
State
(902MB and
903MB Only)
Default=
65535,
Disabled.
The four lower order bits of
this 16-bit register value
define the state the output
channels of the port will be
programmed to following a
watchdog timeout. Bit 0
corresponds to channel 8,
bit 1 to channel 9, bit 2 to
channel 10, and bit 3 to
channel 11. Write 65535
(FFFFH) to this register to
leave the outputs
unchanged following a
timeout (this is also the
default value).
4
0010
9
(0009)
Response
Delay Time
(Turnaround
Delay)
Default=0,
No Delay
Can be set from 0 to 65500
ticks (1 tick = 1.085us).
This is the additional delay
the module will wait before
responding to a message
from the host. Increase
this value if you have
trouble communicating with
the module or you
encounter a high degree of
error messages.
4
0011
10
(000A)
Alternate
Method of
Module
Reset
Writing 41429 (A1D5H) to
this register will cause a
module reset. This is
provided as an alternate
method of Reset for
software that does not
support the Reset Slave
(08) command.
After a reset, this register
reads 0 (no reset).
This register is not
maintained in EEPROM.
41001
.
.
.
This block
mirrors
1xxxx
registers.
Refer to Note 3. 1xxxx
Discrete Input Registers
are mapped to the 41xxx
Holding Register space
using an address offset of
41000.
42001
.
.
.
This block
mirrors
0xxxx
registers.
Refer to Note 3. 0xxxx Coil
Registers are mapped to
the 42xxx Holding Register
space using an address
offset of 42000.
43001
.
.
.
This block
mirrors
3xxxx
registers.
Refer to Note 2. 3xxxx
Input registers are mapped
to the 43xxx Holding
Register space using an
address offset of 43000.
Notes (Register Map):
1.
Configuration variables stored in holding registers (4xxxx
reference addresses) are maintained in EEPROM except as
noted. Changes to these register parameters do not take
effect until the next software or power-on reset of the
module.
Notes (Register Map)…continued:
2.
For your convenience, this module mirrors the contents and
operation of 3xxxx registers into 43xxx holding register
space for systems and controllers that cannot directly
access 3xxxx registers. That is, the 3xxxx registers of this
model can be written to, or read from, using either the
standard methods described in the Modbus specification, or
through mapping (mirroring) to the Holding Register space.
The format of the registers are identical and you only need
to offset your address by 43000. For example: if you want to
read Input Register 1 through the Holding Registers, you
would use the “Read Holding Registers” function with an
address of 43001.
3.
For modules with a firmware revision later than G (as in
9300-029G of 901MB, 9300-030G of 902MB, and 9300-
031G of 903MB), the mirroring function as described in Note
2 is augmented as follows (0xxxx also maps to 42xxx space,
and 1xxxx also maps to 41xxx space):
For 1xxxx Input Status Registers (where supported), the
return data is reformatted to match the Holding Register
format. For example, if you request Input Status for 12
digital inputs, instead of getting 2 bytes returned with the
first 12 bits representing 12 digital inputs, you will get 12
separate words (2 bytes) instead, with each set to 0000H
(OFF) or FFFFH (ON).
For 0xxxx Coil Registers (where supported), reads are
handled in the same way as noted for 1xxxx Input Status
Registers. That is, you may write to the coil registers using
the “Preset Single Register” function with an address offset
of 42000. Setting the data to 0000H will turn the coil OFF,
while setting the data to FF00H will turn the coil ON.
AN INTRODUCTION TO MODBUS
The Modbus protocol provides an industry standard method
that Series 900MB modules use for parsing messages. Modbus
devices communicate using a master-slave technique in which
only one device (the master) can initiate transactions (called
queries). The other devices (slaves) respond by supplying the
requested data to the master, or by taking the action requested in
the query. The Series 900MB modules are slaves, while a typical
master device is a host computer running appropriate application
software. Masters can address individual slaves, or can initiate a
broadcast message to all slaves. Slaves return a response to all
queries that are addressed to them individually, but no response
is returned to broadcast queries from a master device.
Remote Terminal Unit (RTU) Mode
The Series 900MB modules use the widely accepted Modbus
network protocol in the RTU (Remote Terminal Unit) serial
transmission mode. In RTU mode, each 8-bit message byte
contains two 4-bit hexadecimal characters, and the message is
transmitted in a continuous stream. The format for each byte in
RTU mode is outlined below:
RTU Mode Byte Format
Coding System
8-bit binary, hexadecimal 0-9, A-F, two
hexadecimal characters contained in each
8-bit field of the message.
Bits Per Byte
1 start bit + 8 data bits, lsb sent first + 1bit
for even/odd parity or no bit for no
1 stop bit if parity is used, or 1 or 2 stop
bits with no parity.
Error Check Field
Cyclical Redundancy Check (CRC)
