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FINE–TUNING AN EMBEDDED IDEA

Part 3: Armed and Ready
by Fred Eady

Start ý Retro Rabbit ý Preflight Checklist ý Firmware Developmentý Left Turn, Clyde• Successý Sources and PDF

LEFT TURN, CLYDE

As I was finishing up the upgrade module firmware, the customer threw in an additional requirement. The upgraded module should be able to operate standalone and dial up a remote plant floor master. The modem hardware was not to be included with every module, however, each module should have the capability to be attached to the modem hardware. Fortunately, the specification stated that the RS-485 and modem functions be mutually exclusive. That meant that I could use the second serial port for the modem interface as well as an RS-485 port.

As of this writing, I did not have the professional modem boards back to show you. However, I followed the same baseline technique I used with the RCM2300 Development Kit and the serial circuits on the upgrade module. I built the development prototype in Photo 7, which allowed me to connect the Cermetek CH2100 modem directly to a personal computer serial port or the upgrade module by simply changing the three jumpers.

Photo 7ýThe 276 part number on the perfboard says Radio Shack, but the dominant component is a full-blown Cermetek 2400-bps modem.

 

The Sipex SP233 on the modem module is there to interface the personal computer serial port to the CH2100. The RCM2300-based upgrade module and the CH2100 donýt need an RS-232 converter between them. Instead, the RCM2300 and CH2100 use standard TTL levels to communicate with each other. The CH2100 is capable of 2400 bps and uses the standard Hayes AT command set. The command set responses replace the hardware modem signals and, thus, reduce the CH2100 pin count to eight. This allows the CH2100 to be directly connected to microprocessor UARTs with no additional supporting parts.

Two wires, power, and a phone jack are all thatýs required to place a data call with the CH2100. The interface for the modem or RS-485 lines are supplied by a 5-pin header of solder connection directly to the right of the EDTP symbol on the upgrade motherboard. Screw terminals on the modem module allow for quick installation in the field.

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