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the interface assumes that the client or network has experienced some sort of unexpected problem, and will close that socket.
•Because the socket timeout determination is performed on a per-socket basis, note that a certain degree of caution must be exercised when using the network timeout feature to avoid “nuisance” timeouts from occurring. Specifically, do not perform inadvisable behavior such as sending a request from the master device to the interface, and then closing the socket prior to successfully receiving the unit’s response. The reason for this is because the interface will then experience an error when attempting to respond via the now-closed socket, which will immediately trigger the timeout action. Always be sure to manage socket life cycles “gracefully”, and do not abandon outstanding requests.
•If a socket timeout occurs (regardless of whether it was due to a communication lapse or abnormal socket error), the driver will trigger a timeout event as described in section 10.7.5.
13.1.2 Coil & Discrete Input Mappings
The Modbus TCP driver provides read/write support for coils (0X references) and read-only support for discrete inputs (1X references). These will collectively be referred to from here on out as simply “discretes”. Accessing discretes does not reference any new physical data: discretes are simply indexes into various bits of existing registers. What this means is that when a discrete is accessed, that discrete is resolved by the interface into a specific register, and a specific bit within that register. The pattern of discrete-to-register/bit relationships can be described as follows:
Discrete 1...16 map to register #1, bit0...bit15 (bit0=LSB, bit15=MSB) Discrete 17...32 map to register #2, bit0...bit15, and so on.
Arithmetically, the discrete-to-register/bit relationship can be described as follows: For any given discrete, the register in which that discrete resides can be determined by:
⎢ discrete + 15 | ⎥ | Equation 1 |
register = ⎢ | 16 | ⎥ |
⎣ | ⎦ | |
Where the bracket symbols “⎣ ⎦” indicate the “floor” function, which means that any fractional result (or “remainder”) is to be discarded, with only the integer value being retained.
Also, for any given discrete, the targeted bit in the register in which that discrete resides can be determined by:
bit = (discrete −1) %16 | Equation 2 |
Where “discrete” ∈[1…65535], “bit” ∈[0…15], and “%” is the modulus operator, which means that any fractional result (or “remainder”) is to be retained, with the integer value being discarded (i.e. it is the opposite of the “floor” function).
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