Omega A2400 - Manual (Page 18)

Communication 3-2
improper command prompt or address is transmitted. The table below lists
the timeout specification for each command assuming that delay times T1,
T2, T3 = 0:
Table 3.1 Response Timeout Specifications.
Mnemonic
Timeout
DO, OC, CC, RD, REA, RID, RLP, RS, RSP, RSU,
≤
10 ms
RT1, RT2, RT3, WE
EA, ID, LP, RID, RR, SP, SU, T1, T2, T3
≤
100 ms
The timeout specification is the turn-around time from the receipt of a
command to when the module starts to transmit a response.
Data Format
All A2400 modules communicate in standard NRZ asynchronous data
format. This format provides one start bit, seven data bits, one parity
bit and one stop bit for each character.
RS-485
RS-485 is a recently developed communications standard to satisfy the
need for multidropped systems that can communicate at high data rates
over long distances. RS-485 is similar to RS-422 in that it uses a balanced
differential pair of wires switching from 0 to 5V to communicate data. RS-485
receivers can handle common mode voltages from -7V to +12V without loss
of data, making them ideal for transmission over great distances. RS-485
differs from RS-422 by using one balanced pair of wires for both transmitting
and receiving. Since an RS-485 system cannot transmit and receive at the
same time it is inherently a half-duplex system. RS-485 offers many
advantages over RS-232:
1) balanced line gives excellent noise immunity
2) can communicate with modules at 38400 baud
3) communications distances up to 10,000 feet.
4) true multidrop; modules are connected in parallel
5) individual modules may be disconnected without affecting
other modules
6) up to 32 modules on one line; 124 with repeaters
7) no communications delay due to multiple modules
8) simplified wiring using standard telephone cable
An RS-485 system usually requires an interface such as the A2400 to
convert RS-232 to RS-485.