Beside the network interface, the ACU also provides an RS232 serial port which can be used to control the device remotely. Depending on the device address set, the ACU either runs framed protocol with start/stop characters and checksum or it provides a dumb terminal interface. The RS232 interface always operates at 9600 baud, no parity, 8 data bits, one stop bit.
This interface is not yet implemented in the ACU19 and ACU-RMU! At these versions, you have to select ‘NONE’ to enable the function of optional frontpanel display and keyboard. If another setting is selected, an optional frontpanel and keyboard has no function.
If an address ‘A’ .. ‘G’ is selected, the ACU expects each message it receives to be packed into a frame as described below.
| char # | example | description |
|---|---|---|
| 1 | { | start character, always ’ {’ |
| 2 | A | device address (A..G) |
| 3 | l | first character of the message body |
| . | e | message body … |
| . | v | .. |
| . | l | .. |
| . | = | .. |
| n-1 | ? | last character of the message body |
| n .tc} | end character, always ‘}’ | |
| n+1 | . | checksum |
The checksum byte is calculated using an algorithm as implemented by the following formula:
This protocol type is known as MOD95- or Miteq protocol . The ACU also packs its reply in a protocol frame as described above. Incomplete frames, checksum errors or address mismatches let the ACU ignore the message. The time between the characters of a message must be less than 5 seconds or the ACU will treat the message as incomplete.
If the ACU is set to the device address ‘NONE’, it uses a simple line protocol instead of the framed protocol described above. Messages sent to the ACU have to be terminated with a carriage return character (ASCII 13), the ACU terminates replies with a CR/LF pair (ASCII 13/10). There is no echo for characters entered, hence this protocol easily may be used for computer based remote control. This setting is also used for allowing communication to an optional frontpanel display and keyboard if implemented.
If ‘TERM’ is selected the serial interface is used to allow communication between the ACU and a sat-nms Handheld. This function is not available at ACU-RMU and ACU19 Version.