Micromod Micro-DCI: 53MC5000 PLC AND PRINTER INTERFACES - Manual (Page 77)

PLC Read Control Bytes (Table 6-4) (Cont)
Function Code (B669, B645) - This code defines the type of value to be read from
the PLC (e.g., coil, status, holding register, input register)
PLC Write Control Bytes (Table 6-5)
PLC Address (B674, B650) - This is a decimal value of the PLC address.
Starting PLC Memory Address Low and High Bytes (B675, B651, B676, B652) - Also,
see Section 3.4 to ensure these values are properly calculated.
Number of L-bytes to Write (B677, B653) - Can not exceed 64.
Number of C-words to Write (B678, B654) - Can not exceed 64.
Function Code (B679, B655) - The destination code for coils or holding registers.
APB Setup Bytes (Table 6-2)
Mode (B290, B456) - 3 to start the Modbus RTU PLC interface protocol.
RS-232/485 ITB
XMT (CR13) and RCV (CR14) LEDs alternately blink whenever there is PCS-PLC
activity.
6.4.16 PCS MODBUS MASTER FAULT ISOLATION AIDS
Table 6-6 summarizes information provided in this section and other sections of the book that can
be referenced as an aid to fault isolation.
Table 6-6. PCS Modbus Master Fault Isolation Aids
Environmental/Power
See Table 1-1 for RS-232/485 ITB environmental and power specifications; see 53MC5000 PCS
Instruction Bulletin for PCS environmental and power specifications.
PCS Setup Errors
Setup Errors 1 through 4 (Table 6-3) - Violating memory map restrictions. Setup Errors 5 and 6 =
Invalid Function Code and Bad Write Command. Setup Error 10 - Scan Time at 0. PCS APB
Setup bytes (Table 6-2) - Should agree with the communication setup of the PLC system.
Scan Overrun Counter (Table 6-3 and Section 3.7) - Expand Scan Time if the count increases at
an unacceptable rate.
Communications
Proper cable fabrication between the RS-232/485 ITB and the PLC (Figures 6-2 and 6-3).
Communication Error and Error Count (Tables 6-4 and 6-5) - For checksum (Block Check
Character [BCC]) and bad message errors, the PCS does not use the data but will try again, for
example: if a checksum error occurs during the read, the PCS will perform the write in that Scan
Time and attempt another read the next Scan Time. If there are five consecutive read errors or
five consecutive write errors, the PCS causes a re-initialize sequence.
PCS Master only - Execute the Diagnostic command (Function code = 8) and the Return Query
Data Diagnostic Code 00 00 (Loopback Test) to test the communications link.
Possible PLC Problems
PLC Error Code and Error Count (Tables 6-4 and 6-5) - Reference the appropriate PLC
documentation for the required action indicated by the returned Exception Response Code.
Execute the Diagnostic command (Function code = 8) and the Return Diagnostic Register
Diagnostic Code 00 02 to determine if there are any PLC hardware failures. Reference the PLC
documentation for the Diagnostic Register error code bit assignments by product model.
1 of 2
Section 6. MODBUS RTU Mode
MODBUS2
6-13