On the Honeywell side:
1) Serial configuration - word size & parity
The only serial format 7800 modules support is for an 8 bit word, one start bit, one stop bit, no parity (8-N-1).
There are no settings for parity because the module only supports no parity, one stop bit.
It is imperative that the Modbus master be set for 8-N-1.
2) Baud rate:
S7800A baud rate is configured via keypad (see manual, search for MB Baud).
The default baud rate is not stated, but I believe it is 9600 baud. (choices: 9600, 19.2K or 38.4K)
The Modbus master must be set at the same baud rate.
S7810M baud rate is set with a jumper to either 9600 or 19.2K. Default is 9600 baud.
Make sure that the Master (PLC) is set to the same baud rate as the module.
3) You must assign a slave address to the keyboard/Modbus module.
The default address, 00, enables the Honeywell proprietary serial protocol, Controlbus.
Addresses 01 and higher enable Modbus.
The slave address is assigned on the S7800A by keyboard entry.
The slave address is assigned on the S7810M by two rotary switches (one for 10's, one for 01's) on the bottom.
4) On the S7810M, you need to wire the RS-485 to the correct 3 (of the 8) wiring terminals.
5) As with all RS-485, the A/B, (data +)/(data -) terminals are arbitrarily assigned by the manufacturer.
If reversed or backwards, with respect to the Master, no RS-485 communication will occur, but the line drivers will not be damaged.
Swap wires on one end if there's no communications to see if the wiring was reversed.
6) As is the case with Modbus, there is a one offset between the interpretation of addressing starting at 0000 and register numbering starting at (4)0001.
Hence it pays to experiment, trying 0000 or (4)0001 when there is a question about getting the correct register.
7) Honeywell recommends 120 ohm terminating resistors at the ends of the RS-485 link.
A working connection can usually be made in close, local (benchtop) conditions, without the resistors.
8) I believe the 7800 Modbus modules must be plugged into a 7800 controller in order to have viable data.
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The Fault Code is in register (4)0001. It is an unsigned 16 bit integer. A table in the manual describes the fault codes.
(4)0014 has the First Out code for the S7830 annunciator. The S7830 is a separate unit that communicates over Controlbus to the 7800 controller. If you don't have an S7830, there won't be anything in (4)0014.
If you do have an S7830 annunciator, then you need an S7810M module to talk Modbus and Controlbus simultaneously. The S7800A can talk only one of the two protocols, not both.
I have no idea what 7153 is. The register maps for the S7800A and the S7810M do not have a 7153 register.
