X8 Register 274

X8 and IO12 register 274

Register 274 is the serial Interface mode configuration register for both X8 as well as IO12. The functionality is documented in the X8 manual, however there are some special bits which are not extensively documented in the manual. The following information applies:

Bit 0: 9600 (set) or 19200 (reset) baud
Bit 1: reserved
Bit 2: reserved
Bit 3: direct I/O (set) or debounced (reset)
Bit 4: “sticky IO” (set) or standard (reset)
Bit 5: IO/Change indication (set) or standard Modbus
Bit 6: Wiegand (set) or standard I/O
Bit 7: No (set) or even (reset) parity.

Bit 7 and 0 are self-explanatory and also documented in the manual. For an X8 only (NO IO12), setting bit 6 enables "Wiegand" mode. If set, bit 7 and 6 will serve as a Wiegand interface, and an Access Control reader or keypad can be connected to the X8 on these pins. The X8 will receive wiegand data from the interface and make it available via the modbus interface.

Bit 3: Direct I/O. If set, I/O is not debounced as usual, but directly reported with a register read.

Bit 4: Sticky I/O. If set, any change in the inputs will "freeze" the I/O into a holding register until the data is read from the host (normal poll/read). That allows to catch small pulses. as long as they don't happen more often than a poll cycle. Example: energy meters - their consumption unit output pulse may be only 100ms wide, but won't happen more often than every second (this is an example). Using Sticky I/O, you know you will catch the pulse even if you read multiple X8's in a poll loop and the loop may take half a second.

Bit 5: IO Change indication: this is a special operation mode which is not strictly Modbus/RTU compliant ! It is not recommended to be used unless you know exactly what you are doing. If a large number of devices (X8, IO12) are connected to a host, you could, for example, use this mode to poll very slowly through the device list. Any device which encounters a change in its I/O will send its Modbus address unsolicited onto the bus if the bus is idle (but this could still mean a collision happens if multiple devices do that at exactly the same time). The Modbus master can recognize the address, or at least that something happened on the bus, and poll the devices. This mode can be combined with Sticky I/O, for example, but again, it is not recommended for general use.