Specifications

Introduction
The D1000 modules has been carefully designed to be easy to interface to
all popular computers and terminals. All communications to and from the
modules are performed with printable ASCII characters. This allows the
information to be processed with string functions common to most high-level
languages such as BASIC. For computers that support RS-232C, no special
machine language software drivers are necessary for operation. The
modules can be connected to auto-answer modems for long-distance
operation without the need for a supervisory computer. The ASCII format
makes system debugging easy with a dumb terminal.
This system allows multiple modules to be connected to a communications
port with a single 4-wire cable. Up to 32 RS-485 modules may be strung
together on one cable; 122 with repeaters. A practical limit for RS-232C units
is about ten, although a string of 122 units is possible. The modules
communicate with the host on a polling system; that is, each module
responds to its own unique address and must be interrogated by the host.
A module can never initiate a communications sequence. A simple com-
mand/response protocol must be strictly observed to avoid communications
collisions and data errors.
Communications to the D1000 modules is performed with two-character
ASCII command codes such as RD to Read Data from the analog input. A
complete description of all commands is given in the Chapter 4. A typical
command/response sequence would look like this:
Command: $1RD
Response: *+00123.00
A command/response sequence is not complete until a valid response is
received. The host may not initiate a new command until the response from
a previous command is complete. Failure to observe this rule will result in
communications collisions. A valid response can be in one of three forms:
1) a normal response indicated by a ‘ * ‘ prompt
2) an error message indicated by a ‘ ? ‘ prompt
3) a communications time-out error
When a module receives a valid command, it must interpret the command,
perform the desired function, and then communicate the response back to
the host. Each command has an associated delay time in which the module
is busy calculating the response. If the host does not receive a response in
an appropriate amount of time specified in Table 3.1, a communications
time-out error has occurred. After the communications time-out it is as-
sumed that no response data is forthcoming. This error usually results when
Chapter 3
Communications