The load values were chosen to emulate realistic situations. Allen-Bradley recommends two 150 ohm resistors, one at each end of the transmission line to reduce reflections. (See Ref Allen Bradley 1770-um022-en-p.pdf)
This summary indicates that the Datalink GW1000 transmits with less than one half the power of the other devices with no load and only about one ninth of the power of the other devices under a heavy load.
About the GW1000
What does this mean to the user of the Datalink GW1000?
- The user cannot rely on the Datalink GW1000 to meet the Allen-Bradley distance versus baud rate specifications.
- Error rates are a function of signal power and noise power. Clearly the Datalink GW1000 is less resistant to noise induced errors than other devices.
- CRC (cyclic redundancy codes) can detect many errors, but not all errors.
- Any undetected error, even a single bit, in a process control system can have disastrous consequences.
About Data Highway Plus (DH+)
Data Highway Plus is a local area networks (LANS) that allow peer-to-peer communication among up to 64 stations, but is optimally used for smaller networks consisting of limited nodes (about 15 maximum). The Data Highway link consists of a trunkline that can be up to 10,000 feet long and droplines that can be up to 100 feet each. Each station is at the end of a dropline. Data can be transmitted at baud rates of 57.6, 115.2, or 230.4K bits.
A Data Highway link implements peer-to-peer communication through a modified token-passing scheme called the floating master. With this arrangement, each station has equal access to become the master. The stations bid for temporary mastership based on their need to send information.
Unlike a master/slave relationship, a floating master relationship does not require the current master to poll each station to grant permission to transmit. Therefore, it provides a more efficient network because there is less overhead per transaction.