RS232/485 Operation and Commands
The RS232/485 interface is user selectable. The modes of operation available are as follows:-
diSP
- Image Display Mode:
In image display mode the display value is sent via RS232/RS485 as raw data in the following
format:
<ESC> IXYYYY
Where:
<ESC>is the ESCAPE character (27 Dec, 1B Hex)
I is the character ‘I’ (73 Dec, 49 Hex)
X is the number of image bytes in ASCII (31 to 38 Hex)
YYYY is the raw, 8 bit display data.
This information is output every display update (approx. 4 times per second - depending upon baud
rate). The number of image bytes sent depends on the number of display digits present. This mode is suitable
only when the receiving unit is produced by the same manufacturer as the PM4.
The most common usage would be to provide a large digit display for wide area viewing which just
mimics the smaller display on the measuring instrument. The large digit displays automatically detect the
image mode data and display the correct value accordingly. The data is in seven segment display image i.e.
Bit 0 is segment A, Bit 1 is segment B etc.
Cont
- Continuous Transmit Mode:
In this mode the display value is continually sent via the RS232/485 interface in ASCII format with 8
data bits + 1 stop bit. Data will be updated at approximately the same rate as the sample rate (approx. 4 times
per second - depending upon baud rate). Refer to the
SERL
function for choices of operation mode in
continuous transmit mode. The format for this is as follows:-
<STX> XYYYY<CR>
Where:
<STX> is start of text character (2 Dec, 02 Hex)
X SPACE (32 Dec, 20 Hex) for a positive value.
X‘-’ (45 Dec, 2D Hex) for a negative value.
YYYY is the display value in ASCII.
<CR> is a Carriage Return (13 Dec, 0D Hex)
e.g.: If the display is showing 123456 then the instrument will send
‘02 31 32 33 34 35 36 0D’ (HEX) to the host.
POLL
- Host Controlled Transmit Mode:
This mode requires a host computer or PLC to poll the instrument to obtain display or other
information or reset various setpoint parameters. Special communications software such as “Telix” is
required when using POLL mode. Data is in ASCII format with 8 data bits + 1 stop bit. When polling the PM4 it
is essential that the command characters are sent with less than a 10mS delay between them. This normally
means that each command line must be sent as a whole string e.g. <STX>PA<CR> is sent as one string
rather than <STX> on one line followed by P etc. If testing using “Telix” or other software this is normally
achieved by allocating a command string to a function key. Whenever the function key is operated the whole
string is sent. The format used is ASCII (8 data bits + 1 stop bit) so, for instance, if address 1 is used then the
string <STX>PA<CR> must be put into “Telix”, or similar program as:
^BP!^M
Where:
^B is the ASCII character for STX
P is the command line to transmit the primary display value
! is the ASCII character for address 1 (33 Dec of 21 Hex)
^M is the ASCII character for CR
A typical format for the host command is as follows:-
<STX>CA<CR> (Standard read etc.)
<STX>CA<CR>N<CR>XYYYY (Set Value Command)
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