Experimental Physics and
Industrial Control System

Davide Marcato <[email protected]> · Thu, 14 Dec 2017 13:58:41 +0100

Hello,

I am writing an asynPortDriver port on top of the modbus EPICS module

and I would like to read the initial values of the modbus registers for

writing functions.

I am currently working with modbus 2-10-1 and asyn4-30 and I am using

the absolute addressing. I noticed that when calling readInt32Array on

the modbus port using modbus function 16, the module always performs a

TCP query but does not return the values, because it enters the

following lines of code:

drvModbusAsyn.c line 1276:
                case MODBUS_WRITE_SINGLE_REGISTER:
                case MODBUS_WRITE_MULTIPLE_REGISTERS:
                case MODBUS_WRITE_MULTIPLE_REGISTERS_F23:
                    printf("write multiple registers\n");
                    if (!pPlc->readOnceDone) return asynError;

and readOnceDone is false. This way I can never read the initial values

of the registers. Is this a know behavior or is it an error?

In the readInt32 function (drvModbusAsyn.c line 878) the variable

readOnceDone is set to 1 after the call to doModbusIO. Is there a reason

for not being the same on the readInt32Array function?

Furthermore, would it be a good idea to avoid performing modbusIO more

than once in absolute addressing mode when using a write modbus function

(eg: 16)? This would keep the behavior consistent with the rest of the

module.

I also noticed that on the rest of the module, when trying to read on a

"output" port (meaning that a modbus write function code is in use) the

module always return the value it read at startup, on the first (and

only) reading. Is there a reason why the values written to those

registers are not cached and the last written value is returned instead

of the value the register had on the first reading? I mean somenthing like:

1 - the registers are read (doModbusIO) and their value is saved to a cache
2 - the user writes to some of those registers

3 - the user tries to read those same registers and the modbus module

returns the last cached value for each register

Thank you,
Davide

--
Davide Marcato
INFN - Laboratori Nazionali di Legnaro
Viale dell'Universita', 2
35020 LEGNARO (PD) - ITALY
Phone: +39 0498068439	

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System