Tech Update - ARCNET transceivers: 485 versus 485D
2 June 2004
Computer/Embedded Technology
News & Events
Products & Solutions
Logistics (Industry)
Information from Electronic Products Design
Frequently a customer will ask, "I need an ARCNET interface module for my EIA-485 DC-coupled network, but should I use the 485 or the 485D?" This is an important issue; if a customer orders the wrong model, the product may not work as expected.
The EIA-485 specification deals with signalling over a multidrop balanced transmission line. Thus, differential line drivers and receivers are used. Both AC-and DC-coupling are common, but Contemporary Controls implements the DC-coupling in two distinct ways.
The 485 option assumes that driver software will put the ARCNET controller chip in 'backplane mode'. To exercise this option, a customer must have a driver capable of using the backplane mode. However, many customers do not have this ability.
The 485D option was designed for customers who have long been using legacy DOS driver software. Such software was written before the introduction of newer generation COM200XX controller chips and therefore cannot activate the backplane mode unique to these chips. To achieve proper results - and spare customers the necessity of rewriting their software - Contemporary Controls' 485D transceiver hardware creates the proper signalling in the absence of software control.
Consequently, in choosing between its 485 and 485D ARCNET interface adapter models, the customer must know what type of software is being used. Often prospective buyers do not have this information. As a general rule, if the application software is not Windows-based, the 485D is needed.
Note that proper EIA-485 signalling is achieved with either transceiver. This means 485 and 485D models can coexist on the same network - although it is rare that such a mix of adapters will actually be found. Active hubs are unaffected. Either 485 or 485D transceivers will function with active hubs equipped with 485 transceivers.
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