Introduction
We recently acquired a Starrett 737 Vertical LCD Scale for the knee of a JET JT830 vertical milling machine. We found that, while having the data was handy, it was very inconvenient to view the LCD readout due to the mounting location of the unit. A remote display seemed like a good idea and so we started shopped for something that might work. We found that Starrett made such a box, but a price tag of over $500 was disappointing. We discovered that Mitutoyo manufactures a very nice remote readout box and it was only a little over $200. Both the Starrett Scale and the Mitutoyo Display claimed "SPC compatible", so we ordered a display with high hopes. Much to our disappointment, however, the SPC protocols are not directly compatible and the Mitutoyo Display would just read "error" when connected to the Starrett.
To address this problem, we developed the Starrett to Mitutoyo Protocol Converter. Data from the Starrett is converted to the Mitutoyo format and then retransmitted enabling the scale and the display to work as together.
Below we have described the the prototype system we developed to address the problem. Source code, plans, and a preprogrammed CPU are available (see below) for those interested in using a similar configuration.
Starrett SPC
The SPC output protocol sent by the Starrett LCD Linear Scale is a pseudo standard RS232-like data stream. The signal levels are standard TTL/CMOS, however. The data stream from the Starrett SPC is a sequence of ASCII characters representing the current positional data and the selected units (inch/mm).
Mitutoyo SPC
The Mitutoyo SPC protocol is considerably different from that of the Starrett. Mitutoyo's SPC protocol consists of a stream of 13 BCD digits with other special characters embedded in the stream to indicate data such as current sign (+-), the location of the decimal point, and the units (inch/mm).
SPC - Not a Standard / CPU Core Selection
Because of the vastly different formats of the Starrett and Mitutoyo SPC data streams (SPC is not a standard), building a conversion box requires more than a simple bit of logic circuitry. The 8051 micro core was selected due the the availability of a high-quality C complier. Most any programmable micro could be used for the conversion, however. Initially, the PIC series of chips were considered due to their high performance, low cost, and small size. Hi-Tech Software in Australia (www.hitech.com.au) now has a high performance C compiler for the PIC processors available, making the PIC series chips even more attractive for these types of projects. This author has found the HiTech 8051 C complier to be an excellent value as well as a top perfomer costing about half as much as some of the other big-name embedded compiler companies.
Converter Hardware Board
Point-to-point Prototype Wiring Making the ConversionThe protocol conversion is performed entirely in C on the 8051 micro and takes advantage of the processor's internal serial port for receiving the SPC data from the Starrett. Input from the Starrett SPC is received and stored in a internal RAM array via a high-priority interrupt handler.
A 'Data Request' input from the Mitutoyo Display is continually polled in a software loop. When a request is received, a Mitutoyo formated SPC data stream is created from the current data in the RAM array and then serially clocked out to the Display via two output pins on the 8051.
Using the Remote Display
In operation, the data on the Mitutoyo Display is updated about 3 times per second and is more than adequate for most tasks. Having the large, LED remote display on the knee scale of the vertical mill has proved very useful.
Plans and Source Code Availability
This is a complete diagram of the Starrett and Mitutoyo pinout and protocol descriptions as well as a detailed wiring diagram for the circuit board described above. File is in Adobe PDF format.
Complete C source code is available for the converstion controller as described above. Included in the package are all C source files, header files, and the Makefile preconfigured for use with the HiTech 8051 compiler. Modifications for other processors or compilers should be simple. Also included is the precompiled Intel Hex file for direct download into an 8751.