Going Wireless
Wired Connection
The telemetry industry has standardized on a number of different protocols to use in these types of applications. Most protocols were based upon the assumption that the cabling between the monitoring station and the RTU/PLC is an RS-232 or RS-422 serial link. The protocols commonly used on these serial links are MODBUS-RTU, MODBUS-ASCII, DF1, DNP-3, and IEC870. All of these protocols can operate using hard-wired connections. Because the Raveon Radios mimic a hard-wire (data-in equals data-out), in most cases, the protocols will also work using a wireless modem.
M5 and M7 Modems with Modbus
Raveon radio modems support Modbus-ASCII networks with no special configuration. Modbus-ASCII was designed specifically to work well over wired and wireless modems, and uses 7-bit data. All Raveon modems support 7-bit data.
Modbus-RTU uses 8-bit data. Some modems and older systems do not work with 8-bit data, but Raveon’s wireless modems support both 7 bit and 8 bit data. There are some considerations when using radio modems with Modbus-RTU:
Latency The difference between M5 and M7 series wireless modems and a multi-drop wired network is that the wireless modems introduce some additional latency (delay) into the system. Most Modbus-RTU applications can tolerate this latency, but some cannot. If your Modbus application does not tolerate latency, then use Modbus –ASCII. Modbus-ASCII is compatible with Raveon radio modems. The following table shows Latency vs. Over-the-air bit rate for Raveon narrow band radio modems in the packetized mode.
| Bit Rate | ATR2 Setting | Latency (Seconds) |
| 800 (2L) | 0 | 0.8-0.9 |
| 1200 (2L) | 1 | 0.5-0.6 |
| 2400 (2L) | 2 | 0.3-0.4 |
| 4800 (2L) | 3 | 0.2-0.3 |
| 5142 (2L) | 7 | 0.2-0.3 |
| 8000 (4L) | 4 | 0.2-0.3 |
Time-Outs Some versions of the Modbus protocol have short response timeout requirements that may not be compatible with radio modem latencies. Modbus-RTU is compatible with the normal FireLine latencies but does have inter-character delay requirements that must be met. Raveon modems have programmable time-outs to facilitate the control of latency.
Modem IDs The M5 and M7 series modems have 16 bit IDs. Most SCADA systems work in a broadcast configuration, where all modems hear all other modems. To do this, set the net mask to all zeors (ATMK 0000). Be sure to set each unit ID in each modem to a unique ID number, so that the duplicate packet filtering works properly. All Raveon modems filter out duplicate packets, so that operation with repeaters does not cause duplicate packets being received.
For lowest latency, Raveon’s unique “Streaming” mode of operation provides data transfer with latency only slightly higher than wired configurations. No other radio modem on the market offers both error-free packetized operation AND Streaming data operation.
M5 and M7 Modems with DF1
The DF1 protocol works well with the Raveon radio modems as long as the over-the-air data rate is set to 4800 bps or higher. The stock-configuration of the radio modem works with the Rockwell “DF1 Polling Driver”.
To reduce latency in the polling, it is suggested that certain stock-parameters in the FireLine be a adjusted to values more optimized for use in a polled environment. The following is a list of parameters in the radio that may be adjusted to reduce latency when using the DF1 protocol.
1) Reduce the serial-port time-out value down to 2mS (ATR3 2)
2) Set the serial port to 19200bps (ATBD 4)
3) Configure the Over the air data rate to 8000bps (ATR2 4) This will reduce the communication range, so only do this if the link-margin on the system is adequate.
Use the “Streaming Mode” of communications. (ATMT 2) The factory default is the “Packet Mode”, where all data is error checked and sent in packets. The Streaming mode initiates transmissions faster, and sends characters over-the-air as they stream in, but does not check for errors. DF1 is tolerant of noise and over-the-air bit errors, and in most cases works well in streaming mode. In mission-critical or safety situations, packet mode would be more appropriate as it’s data transmission is more deterministic.