Siemens UL 489 Operating Instructions Manual page 172

Modbus RTU Communication settings
For Modbus RTU communication, the following settings must be made in the COM16 module: baud rate, serial transmission configuration,
Modbus RTU address.
Modbus RTU Slave Address
The Modbus RTU communication address range of the COM16 module is 1 through 126.
Modbus RTU address 0 is used as a broadcast address in Modbus RTU systems.
Modbus RTU address 0xF1(hex) is defined as a broadcast address for COM16 modules.
The Modbus RTU slave address is assigned to data point 5 and may be changed by writing a new address value to high-byte of register
40962 (0xA002). If the COM16 module receives an invalid slave address value, the invalid value will be ignored.
Baud Rate
Baud rate settings of 1200, 2400, 4800, 9600 and 19200 are supported. 19200 baud is the default setting. The baud rate is assigned to data
point 427 and may be changed by writing a new baud rate value to high-byte of register 40984 (0xA022). The following numbers are used to
identify the selected baud rate. If the COM16 module receives an invalid baud rate, the invalid baud rate will be ignored.
Number
0
1
2
3
4
Parity
Parity settings of "No Parity", "Odd Parity" and "Even Parity" are supported. "Even Parity" is the default setting. The parity is assigned to data
point 428 and may be changed by writing a new parity value to the low-byte of register 40994 (0xA022). The following numbers are used to
identify the selected parity. If the COM16 module receives an invalid parity, the invalid parity will be ignored.
Number
0
1
2
Auto configuration of baud rate and parity
The factory settings for baud rate and parity are 19200 baud and "Even Parity". These settings may be changed either by writing from the
master to data points 427 and 428 in register 40994 (0xA022) or via the auto configuration process. The auto configuration process only
occurs when the supply voltage is switched on.
When the supply voltage is switched on, the COM16 module monitors the bus activity. If the COM16 module detects bus activity but cannot
receive any valid data, the auto configuration process is started. The module cycles through all combinations of baud rate and parity until it
finds the combination which allows it to receive valid data. This combination is then saved and the auto configuration process ended.
If the COM16 module does not find a combination that allows it to receive valid data after cycling through all combinations, it will adopt the
original settings for baud rate and parity, and the auto configuration process will be ended.
The time required to complete the auto configuration process can be several seconds depending on:
- Baud rate
- How often the master transmits a telegram
- The length of the messages transmitted by the master
- The number of tests required to detect a valid baud rate/parity combination
9 – 70
Baud rate
1200
2400
4800
9600
19200
Parity
No Parity
Odd Parity
Even Parity