D-Link DE-805TP - 10Mbps Ethernet Mini Hub User Manual download pdf (Page 27)

Modbus for Field Technicians

Page 27

12. OLD DEVICE – SLOW PROCESSORS – LIMITED CAPABILITY

Many older devices have old microprocessors that can't do too much work at

once. Often this microprocessor is used to run the device and handle the

Modbus communication.

It is not uncommon to see device with the following limitations.

* You can only read one data point per message. I.e. length must be 1.

* You must have a delay between sending messages.

13. MODBUS ASCII, JBUS, ENRON AND OTHER VARIANTS

There are several variants of Modbus. They are not interoperable. I.e. A

Modbus RTU master cannot read a Modbus ASCII field device.

ASCII – an attempt to make the Modbus message human readable but

encoding the hex value of each byte in ascii. Stupid. Doubles the message

length.

Jbus – Highway robbery. A Modbus RTU variation that allows more than 9999

of each data type to be read. These days most vendors include this in their

RTU drivers so you don’t have to pay extra.

Enron – Came up with a way of carrying other data in the Modbus messages.

They used multiple words to form data objects. Essentially a set of

conventions. Both the client and server must support them.

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D-Link DE-805TP - 10Mbps Ethernet Mini Hub User Manual Page 27