Series 2000 Communications Handbook

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Modbus and EIBisynch Advanced TopicsSeries 2000 Communications Handbook7. CHAPTER 7 Modbus and EI BisynchADVANCED TOPICS7.1. ACCESS TO FULL RESOLUTION FLOATING POINT ANDTIMING DATA (MODBUS ONLY)One of the main limitations of Modbus is that only 16 bit integer representations of data cannormally be transferred. In most cases, this does not cause a problem, since appropriatescaling can be applied to the values without losing precision. Indeed all values displayable onthe 4 digit Series 2000 front panel may be transferred in this way. However this has thesignificant drawback that the scaling factor to be applied needs to be known at both ends ofthe communications link.One further problem is that certain ‘time’ parameters, notably those used for the programmerfunction are always returned over the communications link in seconds. It is possible for longdurations to overflow the 16 bit Modbus limit.To overcome these problems, a sub protocol has been defined, using the upper portion of theModbus address space (8000h and upwards), allowing full 32 bit resolution floating pointand timer parameters. The upper area is known as the IEEE region.This sub-protocol provides two consecutive Modbus addresses for all parameters. The baseaddress for any given parameter in the IEEE region can easily be calculated by taking itsnormal Modbus address, doubling it, and adding 8000h. For example, the address in theIEEE region of the Target Setpoint (Modbus address 2) is simply2 x 2 + 8000h = 8004h = 32772 decimalThis calculation applies to any parameter that has a Modbus address.Access to the IEEE area is made via block reads (Functions 3 & 4) and writes (Function 16).Attempts to use the ‘Write a Word’ (Function 6) operation will be rejected with an errorresponse. Furthermore, block reads and writes using the IEEE region should only beperformed at even addresses, although no damage to the instrument will result in attemptingaccess at odd addresses. In general, the ‘number of words’ field, in the Modbus frame,should be set to 2 times what it would have been for ‘normal’ Modbus.The rules governing how the data in the two consecutive Modbus addresses are organiseddepending on the ‘data type’ of the parameter.7-2 Series 2000 Communications Handbook Part No HA026230 Issue 2.0 Feb-00
Series 2000 Communications HandbookModbus and EIBisynch Advanced Topics7.2. DATA TYPES USED IN SERIES 2000 INSTRUMENTS• Enumerated parameters are parameters which have a textual representation for their valueon the user interface, for example, ‘Auto’ or ‘Manual’, ‘On’ or ‘Off’, ‘SP1’, ‘SP2’,...,‘SP16’, etc. A full list is included in the parameter tables in the previous chapter.• Status words are generally only available over communications, and are used to groupbinary status information.• Integer parameters are those that never include a decimal point, however the instrument isconfigured, and do not refer to a time period or duration. These include such values asthe instrument communications address and values used to set passwords, but not ProcessVariable and Setpoint related parameters, even if the display resolution of the instrumentis set to no decimal places.• Floating point parameters are those having a decimal point (or those which may beconfigured to have a decimal point), with the exception of parameters relating to timeperiods and duration. This includes Process Variable, Setpoints, Alarm Setpoints, etc.• Time Type parameters measure durations, and include Integral and Derivative times,program durations, etc.7.3. ENUMERATED, STATUS WORD, AND INTEGERPARAMETERSThese use only the first word of the 2 Modbus addresses assigned to them in the IEEE area.The second word is padded with a value of 8000 hex.Although ‘Write a Word’ (Function 6) is not permitted, this type of parameter may be writtenas a single 16 bit word using a Modbus ‘Block Write’ (Function 16). It is not necessary toadd a padding value in the second address. Similarly, such parameters may be read using aModbus ‘Block Read’ (Function 3 & 4) as single words, in which case the padding word willbe omitted.It is, however, necessary to pad the unused word when writing this sort of data types as partof a block containing other parameter values.Series 2000 Communications Handbook Part No HA026230 Issue 2.0 Feb-00 7-3
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