Series 2000 Communications Handbook

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Modbus and EIBisynch Advanced TopicsSeries 2000 Communications Handbook7.8. PROGRAMMABLE LOGIC CONTROLLERS AND 2400SERIES INSTRUMENTS7.8.1. EI-BisynchThis applies to instruments with a version number of 3.00 or greater only.EI-Bisynch uses a variable length data field when sending or receiving numeric data. Thismeans that the number of characters expected in response to a request for a parameter value isnot known at the time of making the request. For example, a PV might be returned as 9.87 (4characters, including the decimal point), 99.65 (5 characters), or even -99.99 (6 characters).Many Programmable Logic Controllers use simple ‘Basic’ modules to provide serial comms,which often require that a certain, fixed, number of characters is expected in reply to a requestmade over serial comms. In order to permit use of Bisynch with such modules, it is possibleto configure the instrument to return fixed field length data.The returned data containsleading ASCII space characters which make up the total returned field length to 7 characters.For example‘ 9.87’‘ 99.65’‘ -99.99’(Note that the quote (‘) characters are not returned). This means that an example reply froman instrument for a read of PV would be as follows:[STX]PV9.87[ETX](BCC)i.e. a total of 12 characters.This does not apply to hex format, which always returns a field length of 5 characters(‘>ABCD’).To switch this facility on, write >0001 to the ‘FX’ mnemonic (hex format). The setting ofthis mnemonic is held in non-volatile memory and so you only need perform this operationonce. To cancel the facility, write >0000 to FX.7-8 Series 2000 Communications Handbook Part No HA026230 Issue 2.0 Feb-00
Series 2000 Communications HandbookModbus and EIBisynch Advanced Topics7.8.2. ModbusThere are many ways of connecting 2000 Series Instruments to Programmable LogicControllers using Modbus, for example the ProSoft 3100/3150 MCM module for AllenBradley PLC/5 and SLC/5. It is usually best to avoid the use of Basic modules which mayresult in very slow communications. Your supplier will often be able to advise on a solutionfor a particular make of Programmable Logic Controller, but if requesting information fromthird party vendors, note that the 2000 Series supports standard Modbus RTU, allowing useof function 16 for block write operations, and functions 3 and 4 for reads.Because Modbus modules often allow a restricted number of block operations, it issometimes useful to create large blocks containing all the data to be written for a giveninstrument. Because the 2400 contains a mixture of read/write and read-only data, this can bedifficult to achieve. Therefore, for 2400 firmware versions 3.00 and greater, a facility hasbeen provided that allows block writes to continue even if values in the block are notcurrently writeable (the values that are not writeable are ignored, and there is no error return).To switch this facility on, write a value of 1 to the instrument Modbus register 220. Thesetting of this register is held in non-volatile memory and so you only need perform thisoperation once. To cancel the facility, write 0 to register 220.Series 2000 Communications Handbook Part No HA026230 Issue 2.0 Feb-00 7-9
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