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Modbus/ASCII Tips

Q: Is Modbus/ASCII a separate protocol from RTU?
Q: Is Modbus/ASCII limited to 9999 registers?
Q: Does Modbus/ASCII have a fixed 1 second time out?
Q: Does Modbus/ASCII move any ASCII messages?

Question? Is Modbus/ASCII a separate protocol from RTU?

Answer: Not really. It is just a different encoding.

One hears nonsense about how MB/ASCII is slightly different than MB/RTU, but really it is just an RTU messages encoded a bit different. This means you can implement you 'Modbus/RTU' send routine as:

If( msg.form == RTU)

then send_as_rtu( msg)

else send_as_ascii( msg)

That's all there is to it. You have a binary message, and it is either sent as MB/RTU or as MB/ASCII. When you receive as MB/RTU or MB/ASCII, you end up with a 256-byte or less binary message. There is no reason to allocate 512 bytes for mythical ASCII messages. Just receive the MB/ASCII message 2 hexadecimal bytes at a time and convert them to a single 8-bit binary value.

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Question? Is Modbus/ASCII limited to 9999 registers?

Answer: No, it can handle many message MB/RTU can.

One hears nonsense about how MB/ASCII is limited to 9999 registers or is one (1) based instead of zero (0) based like MB/RTU. But no, you can send any MB/RTU message as either MB/RTU or MB/ASCII. There is no extra limitation to MB/ASCII.

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Question? Does Modbus/ASCII have a fixed 1 second time out?

Answer: Yes and No. You should make it user settable.

The Modbus specification implies Modbus/ASCII must have a fixed 1 second time out. This may be a good starting default, plus setting it shorter than 1 second can cause problems in the situations where one wants to use MB/ASCII. However, the problem with a fixed 1 second time out is when you start sending the message over a wide-area network (WAN). For example if you bridge a Modbus/ASCII master polling a remote slave as Modbus/TCP. Hard limiting the Modbus/ASCII transaction to 1 second will bite you hard in the posterior when you meet a satellite or other WAN technology that routinely sees 1 or 2 second delays. I'd suggest a minimum user settable range of 1 to 5 seconds, but personally I don't see any problem with giving users the same range as you offer them for MB/RTU.

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Question? Does Modbus/ASCII move any ASCII messages?

Answer: No. MB/ASCII can only move hexadecimal characters.

You may run across a few older products that do bad things like move pure ASCII strings like 'JAN-20-1994' within MB/ASCII. Instead, 'JAN' must be moved as the 6 hexadecimal characters '4A414E'. So I suppose we should really can it Modbus/Hex. All MB/ASCII messages must be directly convertible to/from binary using the same simple two hexadecimal bytes to/from one 8-bit data byte.

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