What Datalink Won't Tell You About The GW1000

1. What Datalink Won't Tell You About The
GW1000
Datalink Datalink & General Datalink
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The Datalink GW1000
Does Not Meet Allen-Bradley Data Highway Plus (DH+)
Specifications
The following information is information that Datalink probably does not want you to know about their GW1000.
After performing load tests on the network, it was quite obvious that the design of the GW1000 was not done in
accordance with Allen-Bradley's DH+ 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
A Summary of the four different load tests can be found in the table below. Each of the images above includes
three Allen-Bradley DH+ products, one Equustek DL3500 and a Datalink GW1000.
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2. Table: Peak-to-Peak Voltage of the DH+ port under different load conditions
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).
The recommended Belden 9842 ‘Blue Hose’ cable (See www.belden.com/pdfs/Techpprs/DMX-512.pdf), has a
capacitance of 12.8 picofarads per foot, so 10,000 feet at 57,600 Hz represents an impedence of 22 ohms.
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.
What does this mean to the user of the Datalink GW1000?
1. The Datalink GW1000 cannot be relied on to meet the Allen-Bradley distance versus baud rate
specifications.
2. 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.
3. CRC (cyclic redundancy check) can detect many errors, but not all errors.
4. Any undetected error, even a single bit, in a process control system can have disastrous consequences.
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