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SEMI E4-0699 © SE MI 1980, 1999 13 RELATED INFORM A TI ON NOTICE — The m aterial contained in this Related Information is not an off icial part of SEMI E4 (SECS -I) and is not i ntended to modify or supercede the officia…

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SEMI E4-0699 © SEMI 1980, 1999 12
APPENDIX 1
A1-1 Differences from SECS-I 1980
This appendix describes the major differences between
this version of the standard and the version originally
adopted in 1980.
A1-1.1 Signal Connections — The required voltage
levels for pins 18 and 25 on the 25 pin "D" connector
are now optional.
A1-1.2 Data Rate — The 150 baud data rate has been
made optional.
A1-1.3 Timeout Parameters — The T4 timeout, which
limits the inter-block arrival time of multi-block
messages, has been added. Use of the T3 timeout has
been clarified.
A1-1.4 Time Before Length Byte — The protocol time
with limit T2 is now used while waiting for the length
byte after sending an EOT.
A1-1.5 Block Send Acknowledgment — Any character
other than an ACK received after sending the second
checksum byte is treated as an NAK.
A1-1.6 Illegal Block Lengths and Bad Checksums —
An NAK code is sent for an illegal block length or for a
bad checksum, but only after waiting for the sender to
stop sending by forcing an inter-character (T1) timeout.
A1-1.7 Block Number — The wording describing the
block number has been clarified to say that the value,
zero, is allowed only for single block messages.
A1-1.8 System Bytes — The handling of the system
bytes for secondary messages sent by a host is now the
same as for the equipment. Also, specific functional
requirements have been added for the content of the
system bytes field.
A1-1.9 Duplicate Blocks — A mechanism for the
detection of duplicate blocks has been added.
A1-1.10 Messages — The discussion of message
assembly from blocks has been clarified and expanded.
A1-1.11 Transactions — The discussion of transaction
handling and reply linking has been moved from SECS-
II and expanded.
A1-1.12 Appendix — Parts of the Appendix have been
moved to a new section called Related Information to
distinguish the content from the standard itself. The
general node transaction flow chart has been moved
from the Appendix in SECS-II to the Related
Information in SECS-I.
A1-1.13 Titles — The title of SECS-I has been
changed from "Data Link" to "Message Transfer." This
phrase more accurately covers the content of the
standard and avoids confusion with other uses of the
term "data link." The title of the "Data Link" section of
SECS-I has been changed to "Block Transfer." The
"Data Link Control" portion of the Block Transfer
Protocol has been retitled "Line Control."
A1-1.14 W-Bit — The W-bit is now required to be set
consistently in all blocks.
A1-1.15 Documentation — A section has been added
on the documentation required for compliance with the
standard.
SEMI E4-0699 © SEMI 1980, 199913
RELATED INFORMATION
NOTICE — The material contained in this Related
Information is not an official part of SEMI E4 (SECS-I)
and is not intended to modify or supercede the official
standard. Rather, this information describes possible
methods for implementing the protocol described by the
standard and are included as reference material. The
standard should be referred to in all cases. SEMI makes
no warranties or representations as to the suitability of
the material set forth herein for any particular
application. The determination of the suitability of the
material is solely the responsibility of the user.
R1-1 Miscellaneous Notes on SECS-I
R1-1.1 Layered Protocol (see Section 1.3.1) — The
lnternational Standards Organization (ISO) has
published a model for Open Systems lnterconnection
(OSI). The SECS-I protocol both predates the ISO/OSI
model and is not a true "open system" and, therefore,
does not correspond exactly to the ISO/OSI model. The
SECS-I protocol is a communications interface rather
than a network protocol. However, the SECS-I levels
can be roughly compared to layers 1 through part of
layer 5 of the ISO/OSI model. ISO/OSI layer 1, the
physical link layer, corresponds to the SECS-I physical
link. ISO/OSI layer 2, the data link layer, corresponds
to the SECS-I block transfer protocol. lSO/OSI layer 3,
the network layer, is a function of the host and is not
defined in SECS-I beyond the provision for a bi-
directional flow (see Section 6.2). Similarly, network
management is assumed to be the responsbility of the
host. ISO/OSI layer 4, the transport layer, is covered by
the SECS-I block transfer protocol, duplicate block
detection, and the message protocol. ISO/OSI layer 5,
the session layer, is partially covered by the SECS-I
message protocol.
R1-1.2 Single Timer for T1 and T2 (see Section 5.3) —
A single timer can be used for both the inter-character
timer and the protocol timer, since both are the time
between receiving successive characters and both limits
are never in effect at the same time.
R1-1.3 Stalling (see Section 5.8.2) — The line control
portion of the block transfer protocol has the ability to
delay the acceptance of a data block by not responding
with an EOT immediately after receiving an ENQ. Such
an action by the receiver is called "Stalling." If the
delay exceeds the sender's T2 value, the sender will
send another ENQ. This can be continued depending
upon the sender's setting of T2 and RTY. Such a delay
should be an occasional convenience to accommodate
random short delays in the receiver's ability to accept a
new block and should not be counted upon routinely to
make up for poor response. In particular, the block
transfer protocol should probably have at least two
buffers available for storing incoming blocks. This
allows one block to be inspected by the message
protocol while the next block is being received, thus
allowing a reasonably continuous reception of data. If
both buffers are full and the message protocol is slow in
freeing the buffer for the block transfer protocol, then
the block transfer protocol will start stalling the sender.
If the sender is stalled long enough, it will declare a
send error, which is probably the correct thing to do.
The sender cannot distinguish between a reluctance to
receive and failure in the communications. In either
situation, since no block gets through in a
predetermined time limit, the communications link is
effectively broken. Arbitrarily long delays are not
acceptable in SECS-I.
R1-1.4 Determining the Cause of an NAK (see Section
5.8.5) —The block transfer protocol does not include a
way for a sender to determine the cause of an NAK. If
such information is useful for application purposes, it
must be collected at the receiving end.
R1-1.5 Master Sending a Long Message (see Section
5.8) —When the master is sending a long message, the
slave may be unable to send a block, which may result
in timeouts. It is good practice for the master to
introduce enough delay between blocks so that the slave
has a chance to send a block every few seconds.
R1-1.6 Device Identification (see Section 6.3) —
Although the 15 bits of the device ID can identify
32,767 different devices, the host may find it more
convenient to use the upper seven bits to identify the
type of device such as a spinner or diffusion furnace,
and to use the lower eight bits to identify the specific
device of that type.
R1-1.7 Sending Multiple Open Messages (see Section
7.2.4) —The message protocol algorithms defined in
the standard are capable of inter-leaving the blocks of
any number of open multi-block messages. Since the
receiving protocol is sensitive to the time between
blocks of each message being received, it is proper
procedure for the sending algorithm to alternate
between messages when sending blocks from
interleaved multi-block messages.
R1-1.8 Single Timer for T3 and T4 (see Sections 7.3.2
and 7.4.3) — For a given transaction, a single timer can
be used for both the inter-block timer and the reply
timer, since both are the time between receiving
successive blocks of a message, and both limits are
never in effect at the same time.
SEMI E4-0699 © SEMI 1980, 1999 14
Figure R1-1
RS-232 Isolation Example