SCSI adapter ties computers and peripherals into Ethernet

Computer Design V27 N14 Aug 01, 1988 Pg 27 physical 
  by Tom Williams, Western Managing Editor

AIC-6250 SCSI controller  
AIC-301 Buffer Control  
AIC-6225  (1,7 RLL) Data Separator  [AIC-6225A datasheet]
AIC-011 Serializer / Deserializer (SERDES).  [one page only :( ]
12 MHz 8051 CPU  

A new, flexible LAN interface unit lets any device using the small computer system interface (SCSI) connect to any Ethernet network. The SCSI-based Nodem from Adaptec (Milpitas, CA) acts as an Ethernet interface for systems that already have SCSI. The connecting device can be a peripheral-such as a disk drive or a printer-or a computer system. When the device is a computer, network-based applications can be easily ported to Ethernet or written specifically for Ethernet by using protocols to Ethernet's datalink layer of the Open Systems Interconnection model.

Since SCSI supports eight devices per controller, a Nodem link to Ethernet  can be daisy-chained with other SCSI devices such as disk or tape drives. The SCSI sees the Nodem as just another device on the SCSI bus.

Low-cost interface to Ethernet  

Adaptec developed the unit because the company saw a need for a lowcost interface to Ethernet from IBM PC XTs and PC ATs, Apple Macintoshes, minicomputers and so forth, according to Joe Carballosa, Adaptec's LAN product manager. Adaptec has initially targeted its hardware and software product for the Macintosh because it has a built-in SCSI port. The software lets applications written for the Appletalk protocols run unmodified on Ethernet.

Adaptec will also soon offer similar protocols for IBM PCs and compatibles running DOS, as well as for OS/2 and its LAN manager. Any computer with a SCSI adapter and software to access Ethernet will be able to use the Nodem. Initially, Adaptec will adapt popular network software to the Nodem, but third parties can also easily write or adapt applications to the Ethernet protocols.

THE NODEM SCSI-TO-ETHERNET LINK  

The Adaptec Nodem is built from off-the-shelf parts that include a microcomputer, RAM and ROM, and from Adaptec ICs for use in SCSI cont rollers for tape and disk-type peripheral controllers. No specially designed Ethernet ICs are needed. The Nodem can interface to standard Ethernet, Cheapernet or twisted-pair Ethernet.

Adaptec designed the Nodem interface without using any specialized Ethernet silicon. In addition to a microprocessor and memory, the Nodem uses Adaptec's AIC-6250 SCSI controller, AIC-301 Buffer Control,  AIC-6225  (1,7 RLL) Data Separator, and AIC-011 Serializer / Deserializer (SERDES). The SERDES, which is used in SCSI disk controllers to encode sector formats, is flexible enough to encode the data packets Ethernet uses.

* 12 MHz 8051 CPU Microcomputer .
*Optional 16K firmware EPROM slot for on-board Nodem system firmware (for connection of disketteless devices) [Not Shown] 
* 32KB Program RAM for downloadable Nodem system firmware
 

A speed advantage  

Connecting to Ethernet via the SCSI port gives microcomputers like the Macintosh a definite speed advantage. Because SCSI runs at about 8 Mbits/s (1 Mbyte/s) in its asynchronous mode, it's on a fair speed parity with Ethernet and lets systems take nearly full advantage of Ethernet's 10-Mbit/s bandwidth. SCSI II, expected to run at around 4 Mbytes/s in synchronous mode, will have speed to spare. Applications that use Appletalk can run at only 230 kbits/s using Local talk, Apple's physical and data-link architecture. This difference in speed means that 254 Appletalk nodes can be connected via Ethernet, compared to only 32 connected via Localtalk.

In addition, the intelligent nature of SCSI fits well with a network such as Ethernet, which uses CSMA/CD (carrier sense multiple access/collision detect) protocols. Such networks are fairly unpredictable in terms of exactly when a request for access or data will be granted. SCSI's disconnect/reconnect ability lets an interface accept a request and data from the host, then free the host until, for example, the interface has obtained access to the network, has transmitted the packet, and needs more data. The interface then can "reconnect" with the host by issuing an interrupt without the host having to poll it. By the same token, the host doesn't need to get involved with trying to gain access. SCSI also has a target/initiator mode that allows easy peer-to peer communication: any SCSI device can either initiate a connection or be the target of a communication.

Adaptec plans to expand the SCSI to-Ethernet link to include other LAN standards such as IEEE 802.5 token ring, according to Carballosa. Since a single SCSI host adapter can accept up to eight devices, multiple Nodems could be placed on a single host to act as gateways between different network architectures.