Download Enhanced Programmable Logic Controller Gateway Control Functions

2.6.4
(Note (5) Continued)
When the BLOCK MOVE function is used in Modicon controllers, the first (lowest)
addressed discrete location will end up in the register MSB, bit 15, and highest discreet
address in the register LSB, bit 0. Other Modbus devices may not pack in reverse order.
In any case, the EPLCG unpacks bits in the order shown above.
Listed below are other notable points and features for Modbus devices:
•
•
Protocol is Modicon, model type 584 (384 and 484 are also supported).
Port is 1 to 4 for DHP, and 1 to 2 for EPLCG (LCN parameter PCxPORT).
•
Maximum number of registers requested per transaction:
DHP = 32 register values [analog or counter] (=64 bytes), or 64 coils [digitals]
(=4 bytes)
PLCG = 64 register values [analog or counter], or 64*16=1024 coils
[digitals]
•
Error Detection and Retry features on communication link:
(Retry information is based on current documentation, but is subject to change.)
Error description
Message Retried?
CRC-16 error
Yes
Message timeout
Yes
( timeout is 1.5 sec for [E]PLCG, 3.0 sec. for DHP)
Wrong PLC device replied
Yes - for output only
Reply message length incorrect
Yes - for output only
Reply length inconsistent with byte count
Yes - for output only
Modbus Exception Codes:
01
Illegal Function
No
02
Illegal Data Address
No
03
Illegal Data Value
No
04
Failure in PLC (slave) device
No
05
Acknowledge (ACK)
No
06
PLC (slave device) busy
Yes
07
Negative Acknowledge (NAK)
No
Exception codes 01, 02, 03, 04, 05, and 07 will cause the DHP/PLCG point to go
to a BAD state and fail the entire slot to which it’s assigned. This does not cause
failure of the associated device in the DHP box configuration, so points in other
slots will function.
Unless retry is successful, all other faults will cause failure of the associated device
in the DHP box configuration and all associated points and slots, so point data will
not be updated until the fault is cleared. A failure indication will be reported on the
Honeywell system.
•
11-bit character format: 1 start + 8 data + parity + 1 stop = 11 bits.
•
Error checking: Parity (per character) + CRC-16 (per message packet).
•
End-of-message gap is defined as 3.5 character times or more, per Modbus
specification.
EPLC Gateway Control Functions
2-11
9/97