User Manual

32 MHX-920 Operating Manual: Chapter 4 Configuration
S Register 117 - Modbus Mode
Modbus Mode allows for the MHX-920 to be fully Modbus compatible. For
Modbus operation, the general requirement is to get the packet of data to the
receiving serial port with no gaps in the data. The MHX-920 incorporates a
“Modbus Mode” which implements a delay at the receiving modem to
ensure that no gaps are introduced. For most applications, the following
settings are suitable for Modbus operation:
1. Set Modbus Mode parameter S117 = 1
2. Set the minimum packet size (S111) to 1 byte
3. Set the character timeout S116 (rounded to the nearest ms) to roughly
2.5 byte lengths. For example, at 9600 baud,
S116 = 1/9600 x 10 x 2.5 x 1000 = 2.6ms
Rounded up, S116 = 3ms
4. Set parameter S121 = 3 byte lengths. For example, at 19200 baud
S121 = 1/19200 x 10 x 3 x 1000 = 1.56ms.
Rounded up, S121 = 2ms
5. Set S120 as follows:
Slave Side
S120 = (Hop Interval in ms) x (1 + # of Master retransmissions) x (1 +
# of Repeaters over 1)
eg. Hop Interval S109=4 (20 ms),
Master Retransmissions S113=1
Number of Repeaters in system = 2; then,
S120 = 20 x (1 + 1) x (1 + 1) = 80 ms
Master Side
S120 = (Hop Interval in ms) x (1 + # of Repeaters over 1)
6. Try to set the hop interval as short as possible while still ensuring
adequate throughput. It is recommended to set the Hop Interval and
Maximum Packet Size as specified in Appendix E. Performance
Tables. For example, for Master to Slave communication with no FEC,
If a throughput of 60kbps is required, set the Hop Interval S109=3, and
set the Maximum Packet Size S112=110.
The allowable settings for this register are:
*0 Disabled
1 Enabled