In SX Microcontrollers, SX/B Compiler and SX-Key Tool, George Herzog wrote:

I did a little reading and it appears that Modbus wants to use a lot of SRAM - 1k or more bytes.  The PIC18 referred to in the above example can have 4k of registers available while the SX48 [the larger SRAM] has just 262 bytes in SRAM.  So, if you need all those registers for proper operation, you would have to add a memory chip.  I am not sure what all the registers are used for, but Modbus seems to remotely address at least 1k, maybe 4k.  

The sn75176 is driven by the same software for RS-232 serial both in the Modus and in general.  SX/B can easily provide you with USART services in software and these connect via  Tx and Rx pins to the sn75176.  Pins 2 and 3 control the direction of the sn75176 and can be connected together and have on pin control them - recieve would be low and transmit would be high.

In your PIC example, it seems that Driver Enable controls the direction of the sn75176.  If you read the documentation for the sn75176, it explains how to properly use the A and B lines.  This is not the +12/-12 of RS-232, it is differential voltages.  You might read about RS-485 half-duplex to understand it.

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