Not sure if "standard Modbus" is really a thing at all, regardless of what specs may be published or what is deemed "conventional" by some.
I use a tool called Simply Modbus, which is pretty flexible, and the best of a bad bunch of Modbus test software that I've been able to find and try. It does have bugs, and the support I've had from the publisher was dismissive and unhelpful, so please don't regard this as a recommendation - but it might be helpful.