Spectrum Controls 1734sc-IE2CH - Manual (Page 62)

B-2 Point IO™ 2/4 Channel Analog HART Module
User’s Manual Pub. 0300257-01 Rev. A
to its own command messages. The 1734sc-IE2CH or 1734sc-IE4CH module acts as
primary master. A secondary master, such as a handheld configuration device, may also
be connected.
B.1.3 Transaction Procedure
HART is a half-duplex protocol. After completion of each message, the FSK carrier
signal must be switched off to let the other station transmit. The carrier control timing
rules state that the carrier should be turned on not more than 5 bit times before the start of
the message (that is, the preamble) and turned off not more than 5 bit times after the end
of the last byte of the message (the checksum).
The master is responsible for controlling message transactions. If there is no reply to a
command within the expected time, the master should retry the message. After a few
retries, the master should abort the transaction, because presumably the slave device or
the communication link has failed.
After each transaction is completed, the master should pause for a short time before
sending another command, to provide an opportunity for the other master to break in if it
wishes. This way, two masters (if they are present) take turns at communicating with the
slave devices. Typical message lengths and delays allow two transactions per second.
B.1.4 Burst Mode
Burst mode is not supported.
Section B.2
Response Code
and Field Device
Status
Two bytes of status also called the response code and field device status are included in
every reply message from a field or slave device. These two bytes convey
communication errors, command response problems, and field device status. If an error
is detected in the outgoing communication, the most significant bit (bit 7) of the first byte
is set to 1 and the details of the error are reported in the rest of that byte. The second
byte, in this case, is all zeros.
Communication errors are typically those that would be detected by a UART (parity
overrun and framing errors). The field device also reports overflow of its receive buffer
and any discrepancy between the message content and the checksum received.
In RSLogix 5000 software, if the leftmost bit of the ResponseCode is set, it displays a
negative number. In this case, the ResponseCode represents a communication fault.
Change the display format to hexadecimal to interpret communication status.
If the leftmost bit of the ResponseCode is 0 (value 0…127), then there was no
communication error and the value is a ResponseCode from the HART field device.
Response codes indicate if the device performed the command. 0 means no error. Other
values are errors or warnings. To understand the ResponseCode, contact your HART
field device manufacturer or the HART specification.
See Table B-1and Table B-2 for descriptions of the response code and the field device
status.
Table B-1 (Response Codes – HART Device Status Byte 1)
Response Code Description
If Bit 7 is And Bits
6…0 are
1 16#40 Parity Error Vertical parity error - The parity of one or more of the
bytes received by the device was not odd