XpressNET is a high-speed multi-point to multi-point digital network based upon the industry standard RS-485 protocol. DIGITAL plus uses XpressNET to communicate between its high-speed components such as the command station, handhelds, tower cabs and its computer interface. XpressNET communicates at 62.5 thousand bits per second (nearly 4 times faster than its closest competitor). It is the highest performance network in widespread DCC use and all its protocols are freely available to both the industry and the hobbyist.
But isn’t XpressNet a proprietary protocol?
The hardware for XpressNET is based on the industry standard RS-485 protocol which is widely used in industrial control applications
The XpressNET software protocol has been made public by Lenz and provides a complete description including the full protocol suite. You can download the original copy here, alternatively this site provides a local copy of the xpressnet protocol.
RS485 is a very popular worldwide industry standard protocol for communication between high-speed real time devices. It is a multipoint to multipoint network commonly used in industrial real time control applications and networks. RS-485 chips are readily available from a wide variety of sources and like most industry standards, the price/performance of these products keeps getting better. RS-485 can communicate at speeds of up to 10 megabits per second over distances up to 4000 feet. This has been slowed down for model railroad applications to facilitate flexibility for XpressNET installations.
Northcoast Engineering, Ramfixx Technology, Wangrow Electronics, Lenz GmbH, Rivorossi, and ZTC have all independently chosen RS-485 because of its superior performance, commercial availability and reliability.
I have heard talk that RS-485 is a “linear terminated Bus”?
Yes and No. To communicate at the full speed and distance limits of the RS-485 network requires some discipline on the architecture and construction of the network. Electronic Industry Association’s (EIA) RS-485 specification calls for a resistor to be placed at the last node of the network. This is what is meant by termination.
At high speeds, RS-485 is also intended to be wired in a linear fashion, which means you wire it in a point to point fashion rather than a tree with lots of branches.
But because Lenz uses the RS-485 at much slower speeds than the maximum allowed for, these restrictions are not necessary applicable or found in Model Railroad applications.
XpressNET installations can be up to 900m (3000 ft) long using virtually any tree configuration and do not absolutely require a terminating resistor. It is however recommended that a terminating resistor be inserted for installations exceeding 1km.
How fast is XpressNET?
It takes less than a millisecond (1/1000 seconds) to complete a transaction to a handheld that has information to transmit and only 300 microseconds to communicate, should the handheld have no information that needs to be updated. In addition, only active devices that are currently plugged into the network are actually provided the token that allows communication.
This means that you could have a network of 100 handhelds and each user could simultaneously press a speed change and all could be serviced in less than one tenth of a second, guaranteed!
Since only a small fraction of users actually need to transmit information at the same time, the reserve capacity of the XpressNET is actually much much larger. No other DCC model railroad network has such performance and reserve capacity.
Why do some people say that XpressNET is a “slow polled bus” ?
One of the problems present on any dynamic network is how to determine which devices are plugged in, since only active devices need to be provided time windows for communication.
Every few seconds the command station goes out and looks for active devices. This is indeed done by polling all possible devices.
This takes only a few milliseconds every few seconds and is totally transparent to the user.
This polling greatly improves the overall performance of a token based network as devices that are not currently plugged in consume no resources.
But doesn’t the existence of polling prevent true network style operations?
Absolutely not. Polling is simply one efficient technique to find out who is currently active on the network and who is not. Once the identity of the device is known multipoint to multipoint communications are not difficult to achieve. To date most devices on the XpressNET only have need to communicate directly with the command station. We have yet to identify a need for our handhelds to be able to talk directly to other handhelds, but if the need arises, it is easy to provide this capability as well.
If RS-485 only requires 2 wires , why does the XpressNET need 4 wires?
Two wires are for RS-485 and 2 wires are for 12 volt power and ground. These additional wires provide power to the handhelds when they are plugged in. Providing power lines with XpressNET eliminates the need for batteries in our handhelds.
If XpressNET is so fast why only provide support for 30 handhelds?
Good question. The answer is simple. Up to now we have simply had few users who had a need for more than 30 XpressNET devices (most users currently have less than 10). The original EIA RS-485 specification only allows 32 RS-485 loads on a single network which is how we initially came up with that limit. But like CD ROM speeds, the RS-485 chips are constantly improving. All our current devices use 1/4 load chips. This means that RS-485 harwdare used in all our XpressNET devices can currently support 128 devices. Additional improvements in RS-485 chips are happening all the time and 1/8 chips are becoming cost competitive. XpressNET is limited to only 32 devices by software. We may consider increasing the number of devices supported in the future based on market demand. To date we have had little demand for increasing the current limit.
Is all the intelligence in the Command Station?
No. DIGITAL plus distributes the intelligence between all the components on XpressNET . Each handheld has a highly intelligent device and each handheld contains a built-in user replaceable micro controller. This allows the user interface to be maintained in the handheld as well as providing a simple upgrade path towards the future. This style of architecture is called Client Server which simply means that the handheld (Client) needs to communicate with the command station (Server) in order to perform all of its tasks. To avoid the inherent problems of placing batteries in the handheld, the dynamic information the handheld needs to continue processing the user commands is provided to the handhelds by the command station each time the handheld is plugged into the XpressNET .
If XpressNET has so much reserve capacity, why have a separate network for feedback devices?
Feedback devices do not have the same real time performance need as input devices used by model railroad operators. While we could have placed the feedback devices directly on XpressNET network, we decided from the start to use a separate 2-wire bus for this purpose. This has the benefits of lower overall cost, and keeps XpressNET free for more demanding time critical applications such as the Tower Cab and Computer Interface. We see many future devices that can take advantage of the high-speed performance that is inherent in XpressNET . That is why we have avoided the temptation to overload the network now. Preserving your investment is one of our foremost concerns. It is part of the overall plus in DIGITAL plus.
I keep hearing about performance problems in the XpressNET. Why?
We are embarrassed to admit that in our first release of our command station in 1993 we had a minor software problem in our command stations that gave the appearance of slow performance under load. While never a problem with XpressNET , and long since fixed, this software glitch caused significant performance problems in heavily loaded systems. A real advantage of our architecture is that by simply upgrading the software in our command stations whole new possibilities are opened up. It allows us to constantly improve our product while not abandoning any of our customers. Several upgrades to XpressNET protocol and command station software have occurred over the years and all older products continue to work. Customers of our original systems can easily upgrade to the latest software we have available, which provides them many more features than the system had when originally purchased. You can bet that more upgrades to our software are on in the works as the ability to upgrade is one of tenants on which DIGITAL plus has been built.
If all these companies are using RS-485 why can’t I interchange handhelds today?
While all use a common hardware protocol, each currently use’s different communication software. But this is changing. ZTC and Lenz GmbH systems can already interchange handhelds. This is because both companies have adopted a common communications protocol. North Coast Engineering and Lenz GmbH are working toward the ability to support each other’s handhelds. Open protocols based upon industry standards facilities these agreements. Lenz GmbH will continue to support the evolution of these open protocol standards.
Wow, I never knew how much power XpressNET had. What’s in store for the future?
DIGITAL plus continues to evolve and XpressNET will continue to evolve with it. We are very excited about the potential for handheld interchange that an RS-485 based network provides. We will continue to work with other DCC manufacturers to further enhance the powers of XpressNET and we have several new products in the pipeline that even further exploit the power inherent in the existing XpressNET protocol. Simply put XpressNET is fast, efficient and conceptually simple to understand and use. And as the needs arise, we will continue to evolve the protocol to meet those needs.