The Datalink GW1000 Does Not Meet Allen-Bradley Specifications
As can easily be checked by anyone with an oscilloscope, the Datalink GW1000 has sub-standard signal levels.
Image 1: DH+ Network with No Load
Image 2: DH+ Network with 100 Ohm Load
Image 3: DH+ Network with 51 Ohm Load
Image 4: DH+ Network with 10 Ohm Load
In each of the above, three Allen-Bradley devices, one Equustek DL3500 and the Datalink GW1000 are shown communicating on a DH+ Network. Four different load conditions are tested, and are summarized below:
Table: Peak-to-Peak Voltage of the DH+ port under different load conditions
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,GW1000 DH+ ,GW1000 DHP1,GW1000 ABEIP 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 and Datalink (Industrial Automation Company)
Datalink (Also known as Industrial Automation Company,Datalink Technologies Group,Datalink Technologies Gateways,Datalink Systems,Gateway Technologies,Multigate Communication,Advanced Industrial Network) used to make the following false claim about the GW1000: "The serial DF1 to DH+ conversion protocol available in the GW1000-DHP1 is the same one available in the DL3500-DH+/DF1 product".
Another claim they make is the testimonial about the GW1000: "The DL3000 is unbelievable....it keeps on running in the most disgusting, hazardous environment you could ever imagine. We didn't even know what the controller was because of all the messy residue covering the box....it just keeps on working. J.S.". Sometime after the release of the GW1000, it became: "The GW1000 is unbelievable....it keeps on running in the most disgusting, hazardous environment you could ever imagine. We didn't even know what the controller was because of all the messy residue covering the box....it just keeps on working. J.S." Interesting how similar those quotes are.
What does this mean to the user of the Datalink-Industrial Automation Company 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.
Data Highway Plus (DH+) 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.