The HART protocol is an industrial communications protocol that provides 2-way digital communications. The HART protocol can be operated in a point-to-point or point-to-multipoint configuration. The protocol uses a digital FSK signal superimposed onto a 4-20 mA loop. To encode the bits, 0 is defined as 2200Hz and 1 as 1200Hz. Each individual byte of the message is transmitted as an eleven-bit (start bit, 8 data bits, parity, stop bit) UART character at a data rate of 1200 bps.
The Hart protocol uses a Master/Slave implementation. Slaves only respond when queried
Most Digi radios are designed primarily for low data rate, long range communication. All of the Digi radios were designed with a serial interface that typically can operate from 300 - 115.2k and can be programmed to 1200 baud with different data and parity options that are user settable. The serial interface that the radios use is a standard UART interface and is not directly compatible with HART protocol because of special signaling and the 4-20 mA loop physical difference.
There are a number of RS-232 serial to HART converters on the market that will allow the radios to communicate with HART systems. Since HART is a half-duplex protocol, typically these converters need some signal to let them know when they can drive the line. Most of them use the RS-232 RTS signal to control the driver. Changing the CS parameter on the Digi radios can allow the radios to use the CTS line as data frame enable. When this CTS signal is wired to the control on any HART to RS-232 converter it can allow the HART converter to be driven properly at the appropriate times. The CS parameter can allow the CTS line to be driven several different ways allowing for compatibility between a number of different HART converters.
Last updated: Aug 23, 2017