Owners manual

2: Configuring Modbus
Modbus Protocol User Guide 13
Delay after Output of RTS (0-1275 ms, 5ms increments)
Only asked if RTS/CTS mode is variable. After the IAP Device Server asserts the
RTS/CTS signal, it delays from 0 to 1275 ms before continuing. Normally this is set to
0. Only set a value here if your device, modem, or cable is non-standard.
Wait for CTS to Go Active (N/Y)
Only asked if RTS/CTS mode is variable. Answering N causes the IAP Device Server
to ignore the RTS/CTS response from the modem. Answering Y causes the IAP
Device Server to wait for the RTS/CTS response from the modem. Do not answer Y
unless you know that the cable is wired properly to support this signal.
Delay after CTS Going Active (0-1275 ms, 5ms increments)
Only asked if RTS/CTS mode is variable and set to wait for CTS to go active. After
the IAP Device Server sees the modem assert an RTS/CTS response input, it delays
from 0 to 1275 msec before transmitting. If the IAP Device Server waits without
seeing a valid response from the modem, it will return the Modbus exception
response 0x0B (hex) to the Modbus/TCP requesting master.
Delay Dropping RTS after Transmitting (0-1275 ms, 5 ms
increments)
Only asked if RTS/CTS mode is variable. After the IAP Device Server completes
transmission, it delays from 0 to 1275 msec before dropping the RTS/CTS output.
Advanced Modbus Protocol Settings
Changing these parameters takes a bit of thought and planning.
Slave Address (0 for auto, or 1…255 fixed otherwise)
Modbus/TCP includes a Unit ID field, which is used to address multiple Modbus
slaves at a single IP address. Unfortunately, some first generation software drivers
assumed a single slave at each IP and always set the Unit ID field to 0. This causes
the IAP Device Server problems because it requires the Unit ID for the Modbus/RTU
“Slave Address.” To support these older applications, the IAP Device Server allows
you to force a fixed address for Modbus/RTU and Modbus/ASCII, but note that this
restricts you to a single serial slave device per IAP Device Server.
Setting this value to 0 causes the IAP Device Server to use the Modbus/TCP Unit ID
as received. Setting it to any other address causes the IAP Device Server to always
use the set value as a fixed address.
Allow Modbus Broadcasts (1=Yes 2=No)
This relates to the previous issue. The default is 2/No, in which case the IAP Device
Server always assumes a Modbus/TCP “Unit ID” of 0 really means Modbus slave
address 1. Answering No here is like setting a fixed address of 1 (parameter above),
except the fixed address is only used if the Modbus/TCP “Unit ID” is 0.
Note: In the current software version for IAP Device Server, a true Modbus
broadcast is only supported when a serial slave device is attached. A