There are a few fairly simple questions that you should ask which might help guide the decision of choosing a QR or a QB (or a QP ) at the Master and Repeater sites:
Is it more important that this be a low-cost system, or more important that it be a highest-reliability system? A QR at the Master (and Repeater) site does not have the ability to work reliably at a very high transmit duty cycle, while a QB or QP at these sites can transmit constantly without over-heating. If a QR must be used at critical sites due to a limited budget then measures should be taken to ensure the transmit duty cycle is limited to approximately 20%. This can be done fairly simply in most SCADA Hosts by including a brief delay between receiving the response from one Remote and sending a message to the next Remote. There is a way to gather data on the duty cycle in the TView+ Diagnostics software package after the system has been running for some time, or the customer can simply insert a pause of perhaps 200-500 milliseconds between messages, and fine-tune that later if required.
Do you think that your system will have a lot of traffic, or will messages be less frequent? This is a fairly subjective question, and difficult to get an exact answer. But some will tell you "I need to push as much data through as possible" while others may say something like "Oh no, we only poll each site once every 10 minutes and only get a few bytes, so traffic is low." A QB or QP is the clear answer if the first is true, while a QR might be OK in the second case.
Are you able to acquire two radio frequencies, or must you work with only a single channel? If two channels (approximately 5 MHz apart) can be acquired, a QB can be used as the system's collision avoidance Master. This allows the QB to act as a "traffic cop" where it can transmit to tell other remotes to be quiet (remain idle) when it hears a signal coming in from another remote. In this way it is much less likely that two remotes will try to send messages to the Master (or Repeater) at the same time. Such a system reduces the number of collisions, which allows a busy system to operate in the most efficient manner possible. A QP won't get hot, but cannot perform the higher quality collision avoidance as can the QB, because it can only transmit OR receive, never both. But it can operate on a single channel (simplex) if that's all that the customer can get.
Released for:Schneider Electric Canada
