Digitel MPC Manual
Page 45 of 72
3.
When a device recognizes its address, it decodes the message and returns an
acknowledgement to the computer, along with any data that was requested.
Specifics
The serial communications port settings such as number of data/start/stop bits, parity, etc. are
defined in (Table I-1. Specifications DIGITEL MPC.). Every communications exchange
between the controlling computer and a PHI unit using the standard interface consists of a
command packet (sent by the controlling computer), and a response packet (returned by the
remote unit). All characters in these packets are ASCII. All fields are separated by a space (not
required between the checksum and terminator). Hexadecimal numbers can be represented in
either upper or lower case.
Cabling
The MPC functions as DTE (Data Terminal Equipment). When the MPC will be connected to
another DTE (such as a personal computer), a null modem serial cable is required to connect the
devices. The null modem cable swaps the signal and control lines so that receive and transmit
are properly connected. The MPC is equipped with a nine-pin female (DCE) D-sub connector
(Table IV-8, page 61) protruding from the chassis rear.
RS-485 allows communications over greater distances but most personal computers are equipped
with 9 or 25 pin RS-232 serial connectors. RS-485 cables can be as long 1220 meters (4000 ft.)
while RS-232 lines are typically limited to between 15 and 30 meters (50 - 100 ft.). RS-422 is
possible for Apple’s Macintosh computers which utilizes this protocol. Protocol configuration is
accomplished by DIP switch settings according to Table IV-9 (page 63).
Maximum operating baud rate is determined by cable length and the environment near the cable.
External noise sources and line loss degrade the signal. Twisted-pair cable should be used with
signal wires twisted with ground wires. Shielded cables also decrease the maximum
transmission length but may be required for (electrically) noisy environments. The baud rate
should be adjusted (BAUD RATE, page 34) on the MPC to ensure viable communication.
Command packet
The command packet (Table III-7) is made up of at least five fields and contains information
needed for a remote unit to perform a command. The minimum command packet (single
command with no data) is 11 bytes long and consists of the following fields.
<START CHAR>
<space> <ADDRESS> <space> <COMMAND> <space> <CHECKSUM> <TERMINATOR>
1 byte
1 byte
2 byte
1 byte
2 byte
1 byte
2 byte
1 byte
