GS-C200 datasheet(10/31 Pages) STMICROELECTRONICS

10 / 31 page

The general format of a command string is the

following:

ADDRESS

COMMAND

ARGUMENT

CHECKSUM CAR.RETURN

The Address must be the first transmitted charac-

ter and it is present only if the Multipoint protocol is

used (at least one of SEL0, SEL1, SEL2 is different

from zero).

The Command is the second character(s) of the

string, in the Multipoint protocol, but it becomes the

transmission opening character when the Point-to-

Point protocol is used (SEL0, SEL1 and SEL2 = 0).

The Argument, if required, is specified immediately

after the command and its length depends on the

command type.

The Checksum character verifies the correctness

of the received string; its value is determined by the

sum of the binary values of the preceding charac-

ters. The result is cut at the seventh least significant

bit and ORed with exadecimal 10 (C200S/C200

from V2.2) to make the result compatible with the

transmission system. The last character, the string

ending character, is always a Carriage Return that

will be identified in the following by the symbol (

↓).

By connecting the pin CHS (pin 7) to ground, the

checksum character is not anymore requested, and

the task of guaranteeing the correctness of the

message is left to the parity bit. It should be noted

that by using this dialogue mode, the data integrity

confidence level is reduced. Because motion sys-

tems normally operate in manufacturing premises

subjected to heavy electro-magnetic noise, and

because any communication problem may have

catastrophic effects on the system actions, it is a

good practice to use the checksum character when-

ever possible. The checksum character is normally

not used (pin CHS connected to ground) when the

GS-C is connected to a video-terminal, i.e. during

the initial programming and debugging phase. In

the following, three examples of command strings

sent to a GS-C module are given.

Example 1 - MULTIPOINT PROTOCOL. The Host

Computer wants to set the USER output 3 of the

module #2. The command will have the following

format:

10/31

The checksum character 6 results from the binary

sum of the character 2 (ASCII value = 32) + char-

acter A (ASCII value = 41) + character 3 (ASCII

value = 33) truncated at the seventh bit.

Example 2 - POINT-TO-POINT PROTOCOL.

The same instruction is given by the Host to a Point

to Point connected module.

The command will have the following format:

A3t

↓

The checksum character has an ASCII value t that

derives from the sum of the ASCII code A+3 = 41+33

= 74 in binary weighted code or t in ASCII code.

Example 3 - POINT-TO-POINT PROTOCOL

WITHOUT CHECKSUM.

For the same instruction, the command format will

be:

↓

The string consists of command and argument only.

The GS-C feeds back information to the Host every

time it receives a command, therefore it has not to

identify itself to the Host when answering in a

Multipoint connection.

The format of the string answered back by the GS-C

is the following:

ANSW.CODE

ARGUMENT

CHECKSUM

CAR.RETURN

The first character, which always identifies the an-

swer type, may assume one of the following values:

The command string has been correctly

recei ved.

The controller is Busy and cannot process

commands.

The controller is Ready to process commands.

An error has been detected. The type of error

is specified by the number following the ”E”.

A controller status (a position or an USER

input/output status) is sent back and its value

is specified by the characters following the ”V”.

The length of the Argument, present only for ”E” and

”V” answers, can range between 1 and 7 charac-

ters, and it is a function of the received command.

The number following the ”E” code, i.e. the error

argument, specifies the detected error type accord-

ing to the following table:

2A36

↓

Address

Module #2

Command

Activate

Argument

USER out 3

Carriage return

Checksum

GS-C200 / GS-C200S

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