Specifications

Document 12245 Rev. I Sheet 5 of 39
Serial I/O comes as a standard feature. By sending and receiving commands over an RS-232, RS-485
(2-wire), RS-485 (4-wire) or RS-422 port, you can control and monitor most of the SSPA’s functions.
1 Message Framing Format
This framing protocol is used for communications between a host and a unit. This is a polling protocol,
meaning that units answer ONLY when they receive a correctly formatted message from the host. The same
framing protocol is used for all messages to and from a unit. A message from a unit in response to one from
the host is referred to as a reply.
A
C
K
6 bits
0
1
STX
02
COUNT
nn
ADDRESS
aa
. . .
 
MESSAGE DATA
2
CHKSUM
cc
ETX
03
Byte 1 Byte 2 Byte 3 Byte 4 Byte nn-2 Byte nn-1 Byte nn

FRAMING HEADER
 
FRAMING FOOTER
End Body
3
2A
1
The most significant bit of byte 4 is reserved, and should always be 0.
2
The message data can be of any length from 1 byte (6 bits of byte 4) to 250 bytes (249 if End Body byte is used). Byte values 02 and 03
may not appear in the message data.
3
End Body byte (2A) is optional, and is included for compatibility with earlier protocols.
1.1 STX/ETX
All message frames start with the framing byte STX (02) and end with the framing byte ETX (03).
1.2 Count
The second byte of the message frame is the count of all bytes in the entire message, including the STX and
ETX bytes.
The protocol’s framing determines that the smallest valid message possible is 6 bytes.
1.3 Address
The third byte of the message frame is the address. Any unit whose address setting matches this byte will
accept the message.
An address of 00 is used to send a command or control message to all units on the bus.
A reply, if any, will contain the unit’s set address.
1.4 ACK Response Flag
Bit 6 (with bit 0 being the least significant bit, and bit 7 the most significant bit) of the fourth byte in a message
frame is used to request an ACK response. This bit will never be set in a reply.
If an ACK response is returned by a unit, it will be sent quickly following receipt of the last byte of the host
message, if possible. See section 1.8.3, ACK Responses, for specifications.
An ACK response will be returned ONLY if all of the following are true:
1. The address in the host message matches that of the unit. In other words, a unit will send an
ACK response to a message addressed to 0 ONLY if its own address is set to 0.
2. The message is not REJECTED for any of the reasons described under section 1.7.1, Rejected
Messages.
An ACK response is a correctly framed message, with the least significant 6 bits of byte 4 equal to the value
3F.
Example ACK response from a unit whose address is 1: 02 06 01 3F 40 03