Extended Addressing 9-4

Figure 7.1 shows a very simple system but the same addressing methodmay be used to construct very large systems. Each RS-485 string may handle up to 122 addresses, and up to 14884 A2400’s may have unique addresses.

Structured Addressing

Even for a relatively small system, it can be advantageous to employ a hierarchical addressing system as used in Fig. 7.1. This is particularly true in systems that consist of many sites that are identical. From a host software standpoint, each site can be treated identically with the same module addresses, with each site having a different A2400 address.

Extended Address Syntax

The command syntax used with Extended Addressing is quite similar to the normal protocol. The Extended Address commands are initiated with a ‘{‘ character (left curly brace, ASCII $7B), or a ‘}’ character (right curly brace, ASCII $7E). The ‘{‘ prompt is analogous to the ‘$’ prompt in that it returns the shortest possible response to complete the command. The ‘}’ prompt is similar to the ‘#’ prompt in that the command is echoed and a checksum is generated along with the other data necessary to complete the response. The ‘*’ response prompt is used in all command forms.

The Extended Address commands use a two-character ASCII address, each character may be one of 122 legal possibilities. Illegal characters are: NULL ($00), CR ($0D), $ ($24), # ($23), { ($7B), and } ($7E).

Command examples with Extended Address ‘01’:

Command:

{01WE

 

Response:

*

 

Command:

}01WE

 

Response:

*01WE27

 

Command:

{01RS

 

Response:

*31070000

(typical)

Command:

}01RS

 

Response:

*01RS31070000BB

(typical)

Checksums may be appended to commands:

Command: {01WE78

Response: *

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Omega A2400 manual Structured Addressing, Extended Address Syntax