User Manual

MHX-2400 Operating Manual: Chapter 3 Configuration Options 33
S Register 117 - Modbus Mode
Modbus Mode allows for the MHX-2400 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-2400
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